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.

Q: Context: Did you ever make a secret code? One way to make a code is to represent each letter of the alphabet by a different number. Then you can send a coded message by writing words as strings of digits. This is similar to how information is encoded using an electric current. The voltage of the current is changed rapidly and repeatedly to encode a message, called an electronic signal. There are two different types of electronic signals: analog signals and digital signals. Both are illustrated in Figure 23.20. A digital signal consists of pulses of voltage, created by repeatedly switching the current off and on. This type of signal encodes information as a string of 0s (current off) and 1s (current on). This is called a binary ("two-digit") code. DVDs, for example, encode sounds and pictures as digital signals. An analog signal consists of continuously changing voltage in a circuit. For example, microphones encode sounds as analog signals. Electronic components are the parts used in electronic devices such as computers. The components transmit and change electric current. They are made of materials called semiconductors. A semiconductor is a solid crystalusually consisting mainly of siliconthat can conduct current better than an electric insulator but not as well as an electric conductor. Very small amounts of other elements, such as boron or phosphorus, are added to the silicon so it can conduct current. A semiconductor is illustrated in Figure 23.21. There are two different types of semiconductors: n-type and p-type. An n-type semiconductor consists of silicon and an element such as phosphorus that gives the silicon crystal extra electrons. An n-type semiconductor is like the negative terminal in a chemical cell. A p-type semiconductor consists of silicon and an element such as boron that gives the silicon positively charged holes where electrons are missing. A p-type semiconductor is like the positive terminal in a chemical cell. Electronic components contain many semiconductors. Types of components include diodes, transistors, and inte- grated circuits. Each type is described in Table 23.2. Electronic Component Diode Transistor Integrated Circuit (Microchip) Description A diode consists of a p-type and an n-type semicon- ductor placed side by side. When a diode is connected by leads to a source of voltage, electrons flow from the n-type to the p-type semiconductor. This is the only direction that electrons can flow in a diode. This makes a diode useful for changing alternating current to direct current. A transistor consists of three semiconductors, either p- n-p or n-p-n. Current cant flow through a transistor unless a small amount of current is applied to the center semiconductor (through the base). Then a much larger current can flow through the transistor from end to end (from collector to emitter). This means that a transmitter can be used as a switch, with pulses of a small current turning a larger current on and off. A transistor can also be used to increase the amount of current flowing through a circuit. You can learn more about transistors and how they work at this URL: http An integrated circuitalso called a microchipis a tiny, flat piece of silicon that consists of layers of elec- tronic components such as transistors. An integrated circuit as small as a fingernail can contain millions of electronic components. Current flows extremely rapidly in an integrated circuit because it doesnt have far to travel. You can learn how microprocessors are made at this URL: Many of the devices you commonly use are electronic. Electronic devices include computers, mobile phones, TV remotes, DVD and CD players, game systems, MP3 players, and digital cameras. All of these devices use electric current to encode, analyze, or transmit information. Consider the computer as an example of an electronic device. A computer contains microchips with millions of tiny electronic components. Information is encoded as 0s and 1s and transmitted as electrical pulses. One digit (either 0 or 1) is called a bit, which stands for "binary digit." Each group of eight digits is called a byte. A gigabyte is a billion bytes that 
 Question: tiny flat piece of silicon that contains layers of many electronic components

A: integrated circuit
****
Q: Context: Today, nearly 400 animals are listed as threatened or endangered animals in the United States. Many are at risk of becoming extinct or disappearing permanently. Unless efforts to save these vulnerable animals succeed, many of the country's greatest creatures will be lost forever. From the facts listed, guess which animals are endangered. Click on the picture to find out more about each of these animals in trouble. American Crocodile *American crocodiles are about 12 feet long. They live on land and in shallow water, swamps and marshes. *American crocodiles like to float in the water with only their eyes and noses above the water surface. *American crocodiles eat mostly small animals, which they grab with their strong jaws and sharp teeth. *American crocodiles lay eggs. They hide their eggs under twigs and leaves, or bury them in the sand. *Some crocodiles help their young hatch, and then carry them to the water in their mouth. Habitat Southern Florida, Mexico, Central and South American, Caribbean islands. Why It's Endangered Overhunted for its skin and habitat destruction. Peninsular Bighorn Sheep *Bighorn sheep can weigh as much as 280 pounds and stand about 3 feet tall. *They live in dry, desert mountain ranges, near rocky cliffs. *Bighorn sheep eat grass, twigs and leaves. *Male sheep are called rams and can be recognized by their huge, brown horns. The horns curl back over the ears, down,and up past the cheeks. *They live in herds or groups. The male sheep with the biggest horns are usually in charge. Male horns can weigh as much as 30 pounds. *Males will use their massive horns to fight. The fights can last as long as 24 hours. *Females are called ewes. They are smaller than rams and have shorter, smaller horns. Habitat Nevada and California to west Texas and south into Mexico. Why It's Endangered Loss of habitat, hunting illegally, drought and disease. 
 Question: Where can we find both American crocodiles and Bighorn sheep according to the passage?

A: mexico.
****
Q: Context: Biologists agree that the ancestor of mitochondria was an alpha-proteobacterium. But there is no consensus as to what constitutes an alpha-proteobacterial gene. Is it a gene found in all or several alpha-proteobacteria, or in only one? Here, we examine the proportion of alpha-proteobacterial genes in alpha-proteobacterial genomes by means of sequence comparisons. We find that each alpha-proteobacterium harbours a particular collection of genes and that, depending upon the lineage examined, between 97 and 33% are alpha-proteobacterial by the nearest-neighbour criterion. Our findings bear upon attempts to reconstruct the mitochondrial ancestor and upon inferences concerning the collection of genes that the mitochondrial ancestor possessed at the time that it became an endosymbiont. 
 Question: Which species of bacteria did the mitochondria originate from?

A:
biologists agree that the ancestor of mitochondria was an alpha-proteobacterium.
****