Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

[EX Q]: Context: From the outside, the skin looks plain and simple, as you can see in Figure 16.5. But at a cellular level, theres nothing plain or simple about it. A single square inch of skin contains about 20 blood vessels, hundreds of sweat glands, and more than a thousand nerve endings. It also contains tens of thousands of pigment-producing cells. Clearly, there is much more to skin than meets the eye! For a dramatic introduction to the skin, watch this video: MEDIA Click image to the left or use the URL below. URL: The skin is only about 2 mm thick, or about as thick as the cover of a book. Although it is very thin, it consists of two distinct layers, called the epidermis and the dermis. You can see both layers and some of their structures in Figure 16.6. Refer to the figure as you read about the epidermis and dermis below. The epidermis is the outer layer of skin. It consists almost entirely of epithelial cells. There are no blood vessels, nerve endings, or glands in this skin layer. Nonetheless, this layer of skin is very active. It is constantly being renewed. How does this happen? 1. The cells at the bottom of the epidermis are always dividing by mitosis to form new cells. 2. The new cells gradually move up through the epidermis toward the surface of the body. As they move, they produce the tough, fibrous protein called keratin. 3. By the time the cells reach the surface, they have filled with keratin and died. On the surface, the dead cells form a protective, waterproof layer. 4. Dead cells are gradually shed from the surface of the epidermis. As they are shed, they are replaced by other dead cells that move up from below. The epidermis also contains cells called melanocytes. You can see a melanocyte in Figure 16.7. Melanocytes produce melanin. Melanin is a brown pigment that gives skin much of its color. Everyones skin has about the same number of melanocytes per square inch. However, the melanocytes of people with darker skin produce more melanin. The amount of melanin that is produced depends partly on your genes and partly on how much ultraviolet light strikes your skin. The more light you get, the more melanin your melanocytes produce. This explains why skin tans when its exposed to sunlight. The dermis is the inner layer of skin. It is made of tough connective tissue. The dermis is attached to the epidermis by fibers made of the protein collagen. The dermis is where most skin structures are located. Look again at Figure pain, pressure, and temperature. If you cut your skin and it bleeds, the cut has penetrated the dermis and damaged a blood vessel. The cut probably hurts as well because of the nerve endings in this skin layer. The dermis also contains hair follicles and two types of glands. You can see some of these structures in Figure 16.8. Hair follicles are structures where hairs originate. Each hair grows out of a follicle, passes up through the epidermis, and extends above the skin surface. Sebaceous glands are commonly called oil glands. They produce an oily substance called sebum. Sebum is secreted into hair follicles. Then it makes its way along the hair shaft to the surface of the skin. Sebum waterproofs the hair and skin and helps prevent them from drying out. Sweat glands produce the salty fluid known as sweat. Sweat contains excess water, salts, and other waste products. Each sweat gland has a duct that passes through the epidermis. Sweat travels from the gland through the duct and out through a pore on the surface of the skin. You couldnt survive without your skin. It has many important functions. In several ways, it helps maintain homeostasis. The main function of the skin is controlling what enters and leaves the body. It prevents the loss of too much water from the body. It also prevents bacteria and other microorganisms from entering the body. Melanin in the epidermis absorbs ultraviolet light. This prevents the light from reaching and damaging the dermis. The skin helps maintain a constant body temperature. It
Question: __oily substance secreted by glands in the skin
[EX A]: sebum

[EX Q]: Context: The muscular system consists of all the muscles in the body. This is the body system that allows us to move. You also depend on many muscles to keep you alive. Your heart, which is mostly muscle, pumps blood around your body. Each muscle in the body is made up of cells called muscle fibers. Muscle fibers are long, thin cells that can do something that other cells cannot dothey are able to get shorter. Shortening of muscle fibers is called contraction. Muscle fibers can contract because they are made of proteins, called actin and myosin, that form long filaments (or fibers). When muscles contract, these protein filaments slide or glide past one another, shortening the length of the cell. When your muscles relax, the length extends back to the previous position. Nearly all movement in the body is the result of muscle contraction. You can control some muscle movements. However, certain muscle movements happen without you thinking about them. Muscles that are under your conscious control are called voluntary muscles. Muscles that are not under your conscious control are called involuntary muscles. Muscle tissue is one of the four types of tissue found in animals. There are three different types of muscle in the body ( Figure 1.1): 1. Skeletal muscle is made up of voluntary muscles, usually attached to the skeleton. Skeletal muscles move the body. They can also contract involuntarily by reflexes. For example, you can choose to move your arm, but your arm would move automatically if you were to burn your finger on a stove top. This voluntary contraction begins with a thought process. A signal from your brain tells your muscles to contract or relax. Quickly contract and relax the muscles in your fingers a few times. Think about how quickly these signals must travel throughout your body to make this happen. 2. Smooth muscle is composed of involuntary muscles found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels. These muscles push materials like food or blood through organs. Unlike skeletal muscle, smooth muscle can never be under your control. 3. Cardiac muscle is also an involuntary muscle, found only in the heart. The cardiac muscle fibers all contract together, generating enough force to push blood throughout the body. What would happen if this muscle was under conscious or voluntary control? There are three types of muscles in the body: cardiac, skeletal, and smooth.
Question: what muscle pushes food through your intestines?
[EX A]: smooth muscle

[EX Q]: Context: The temperature of the troposphere is highest near the surface of the Earth and decreases with altitude. On average, the temperature gradient of the troposphere is 6.5o C per 1,000 m (3.6o F per 1,000 ft) of altitude. Earths surface is the source of heat for the troposphere. Rock, soil, and water on Earth absorb the Suns light and radiate it back into the atmosphere as heat, so there is more heat near the surface. The temperature is also higher near the surface because gravity pulls in more gases. The greater density of gases causes the temperature to rise. Notice that in the troposphere warmer air is beneath cooler air. This condition is unstable since warm air is less dense than cool air. The warm air near the surface rises and cool air higher in the troposphere sinks, so air in the troposphere does a lot of mixing. This mixing causes the temperature gradient to vary with time and place. The rising and sinking of air in the troposphere means that all of the planets weather takes place in the troposphere. Sometimes there is a temperature inversion, in which air temperature in the troposphere increases with altitude and warm air sits over cold air. Inversions are very stable and may last for several days or even weeks. Inversions form: Over land at night or in winter when the ground is cold. The cold ground cools the air that sits above it, making this low layer of air denser than the air above it. Near the coast, where cold seawater cools the air above it. When that denser air moves inland, it slides beneath the warmer air over the land. Since temperature inversions are stable, they often trap pollutants and produce unhealthy air conditions in cities (Figure 1.1). Smoke makes a temperature inversion visible. The smoke is trapped in cold dense air that lies beneath a cap of warmer air. At the top of the troposphere is a thin layer in which the temperature does not change with height. This means that the cooler, denser air of the troposphere is trapped beneath the warmer, less dense air of the stratosphere. Air from the troposphere and stratosphere rarely mix. Click image to the left or use the URL below. URL:
Question: clouds are in this layer of the atmosphere.
[EX A]:
troposphere