If “proto” means original or primitive, why do we call the protoplanetary disk so?

How air particles exert a pressure on the inside of the balloon

Several kinds of bears are found on earth. most bears are brown or black, but one type of bear, the polar bear, is white. what process led to this difference in fur color? explain your answer.

The mass of a neutron is equal to the mass of a proton plus the mass of an electron. true or false false true