Chemicals called allow neurons in the brain to communicate with one another

the answer is d (apex) good luck : ))

because they are eaten by the frog and the frog getseaten by the snake and the snake by the eagle

sex cells are usually half's then they find another half and make a fertilize egg

they had the law tat factories that produces smoke cannot be built near residential buildings.

building of proper sewage system was done to ensure that the waste is carried away from the humans so that they do not get diseases from it.

also they built water systems to provide water in those areas which are drought prone.