Answer the following question given this paragraph: Muscles can only contract. They can't actively lengthen. Therefore, to move bones back and forth at a joint, skeletal muscles must work in pairs. For example, the bicep and triceps muscles of the upper arm work as a pair. You can see how this pair of muscles works in Figure below . When the bicep muscle contracts, it bends the arm at the elbow. When the triceps muscle contracts, it straightens the arm. Q: Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only do what? Choices: - vibrate - pull - expand - contract A:
contract

Answer the following question given this paragraph: Electrons at the outermost energy level of an atom are called valence electrons. They determine many of the properties of an element. That's because these electrons are involved in chemical reactions with other atoms. Atoms may share or transfer valence electrons. Shared electrons bind atoms together to form chemical compounds. Q: What are electrons at the outermost energy level of an atom are called? Choices: - core electrons - ions - shell electrons - valence electrons A:
valence electrons

Answer the following question given this paragraph: Secondary succession occurs in a formerly inhabited area that was disturbed. Q: What occurs when a former inhabited area gets disturbed? Choices: - tertiary succession - primary succession - tropical succession - secondary succession A:
secondary succession

Answer the following question given this paragraph: Rutherford's experiment showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass of an electron is very small compared to the mass of a neutron or proton, so the electrons in an element do not contribute much to the total mass. The mass number is defined as the total number of protons and neutrons in an atom. Remember that both protons and neutrons have a mass of approximately 1 amu. Knowing the mass number and the atomic number of an atom therefore allows you to determine the number of neutrons present in that atom by subtraction:. Q: According to rutherford, where is the vast majority of the mass of an atom located? Choices: - the cytoplasm - the shell - the electrons - the nucleus A:
the nucleus