What determines the number of covalent bonds that an atom can form

Which of the following is the best definition for the process of photosynthesis? a.) plants digest sugars to make energy. b.) plants use oxygen and glucose to make carbon dioxide. c.) plants use sunlight and carbon dioxide to make sugars. d.) plants use sunlight to make chlorophyll and chloroplasts.

Why do we perceive objects as retaining their brightness under different lighting levels?

Is there any solid scientific evidence that humans have been cloned?