Hydrogen is a likely candidate in the search for a replacement for petroleum. however, removing hydrogen from water requires the investment of a great deal of energy. why are plants considered to be one step ahead in the utilization of hydrogen as an alternative fuel source?

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms

layer 3 is younger than layer 6.

producers within ecosystems are essential as they are at the base of the food chain. plants are examples of producers. they use the process of photosynthesis to capture light energy and use it to convert simple inorganic substances (carbon dioxide and water) into organic substances (glucose).