Convection currents occur: a. Only in liquids
b. When a dense substance is warmer than a less dense substance
c. In both liquids and gases
d. Weakly in solids

answer; motor (efferent)neurons;

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).