What happened that allowed cells to be studied? A. Cells increased in size
B. Scientist began to work together.
C. Scientists became smarter.
D. The microscope was invented and improved.

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.

food source and hunting?

essentially, one bacterial cell splits into two daugurer cells. bacterial offspring don't need or get care from their parents because they are in a sense their own parents. that said, some bacteria can and do depend on other bacteria especially in harsh environments such as bacterial microfilms in hot water tanks.