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 Input:
Context: Elements form compounds when they combine chemically. Their atoms join together to form molecules, crystals, or other structures. The atoms are held together by chemical bonds. A chemical bond is a force of attraction between atoms or ions. It occurs when atoms share or transfer valence electrons. Valence electrons are the electrons in the outer energy level of an atom. You can learn more about chemical bonds in this video: MEDIA Click image to the left or use the URL below. URL: Look at the example of water in Figure 7.1. A water molecule consists of two atoms of hydrogen and one atom of oxygen. Each hydrogen atom has just one electron. The oxygen atom has six valence electrons. In a water molecule, two hydrogen atoms share their two electrons with the six valence electrons of one oxygen atom. By sharing electrons, each atom has electrons available to fill its sole or outer energy level. This gives it a more stable arrangement of electrons that takes less energy to maintain. Water (H2 O) is an example of a chemical compound. Water molecules always consist of two atoms of hydrogen and one atom of oxygen. Like water, all other chemical compounds consist of a fixed ratio of elements. It doesnt matter how much or how little of a compound there is. It always has the same composition. Elements are represented by chemical symbols. Examples are H for hydrogen and O for oxygen. Compounds are represented by chemical formulas. Youve already seen the chemical formula for water. Its H2 O. The subscript 2 after the H shows that there are two atoms of hydrogen in a molecule of water. The O for oxygen has no subscript. When there is just one atom of an element in a molecule, no subscript is used. Table 7.1 shows some other examples of compounds and their chemical formulas. Name of Compound Electron Dot Diagram Numbers of Atoms Chemical Formula Name of Compound Hydrogen chloride Electron Dot Diagram Numbers of Atoms H=1 Cl = 1 Chemical Formula HCl Methane C=1 H=4 CH4 Hydrogen peroxide H=2 O=2 H2 O2 Carbon dioxide C=1 O=2 CO2 Problem Solving Problem: A molecule of ammonia consists of one atom of nitrogen (N) and three atoms of hydrogen (H). What is its chemical formula? Solution: The chemical formula is NH3 . You Try It! Problem: A molecule of nitrogen dioxide consists of one atom of nitrogen (N) and two atoms of oxygen (O). What is its chemical formula? The same elements may combine in different ratios. If they do, they form different compounds. Figure 7.2 shows some examples. Both water (H2 O) and hydrogen peroxide (H2 O2 ) consist of hydrogen and oxygen. However, they have different ratios of the two elements. As a result, water and hydrogen peroxide are different compounds with different properties. If youve ever used hydrogen peroxide to disinfect a cut, then you know that it is very different from water! Both carbon dioxide (CO2 ) and carbon monoxide (CO) consist of carbon and oxygen, but in different ratios. How do their properties differ? There are different types of compounds. They differ in the nature of the bonds that hold their atoms together. The type of bonds in a compound determines many of its properties. Three types of bonds are ionic, covalent, and metallic bonds. You will read about these three types in later lessons. You can also learn more about them by watching this video: (7:18). MEDIA Click image to the left or use the URL below. URL: Chocolate: Its been revered for millennia by cultures throughout the world. But while its easy to appreciate all of its delicious forms, creating this confection is a complex culinary feat. Local chocolate makers explain the elaborate engineering and chemistry behind this tasty treat. And learn why its actually good for your health! For more information on the science of chocolate, see [Link] . MEDIA Click image to the left or use the URL below. URL: 
 Question: force of attraction between atoms or ions that share or transfer electrons

Ex Output:
chemical bond


Ex Input:
Context: Rentrak Corporation (NASDAQ: RENT) is a global media measurement and research company serving the entertainment industry. 
 Question: To which stock exchange does Rentrak belong?

Ex Output:
nasdaq


Ex Input:
Context: On June 5, 1981 the Centers for Disease Control in Atlanta, U.S. announced the presence of a rare, infectious disease in five American homosexuals . Soon doctors were finding similar cases all over the country and the world. The AIDS epidemic had officially begun. By 1983 it was clear that AIDS had spread around the world. More than twenty years later, AIDS has left twenty-six million dead and another forty million infected. AIDS has become the worst medical disaster ever experienced by mankind. Hundreds of young people between the ages of 20 and 45 died each week during the early years of the U.S. AIDS. More than 600,000 cases of AIDS have been reported since 1981 in the United States, and it is estimated that there may be as many as 900,000 Americans infected with HIV. Though the rate of HIV infections continues to increase in the United States, the number of AIDS cases has fallen sharply since 1996, when antiretroviral drugs came onto the market. Unfortunately, the AIDS epidemic continues today in Africa and much of Asia, where antiretroviral treatment is not available and health care is far from enough. AIDS is the fourth leading cause of death worldwide, and the No. 1 cause of death due to infectious disease. HIV, the virus that causes AIDS, was first separated in 1983 by a team at the Pasteur Institute in Paris. That discovery made it possible to develop a test for AIDS and trace back to victims who may have died before doctors were aware of the disease. However, scientists still don't know exactly how and where AIDS started. Finding the source of AIDS could be important in developing a vaccine and be important in mapping the future course of the epidemic. 
 Question: Which areas are the most worrying places where AIDS spreads quickly?

Ex Output:

africa.