When did ferns and conifers evolve? A.Paleozoic
B.after the Cambrian period
C.Precambrian
D.both a and b

Ferns and conifers evolve in Paleozoic era

D. Einstein's theory was difficult for many people to understand.

Explanation:

Einstein’s theory of general Relativity was largely Ignored for three decades because scientists at the time thought the theory was too complex compared to Newton's.

Therefore, it took long before the general science community accepted Einstein's theory of general relativity because Einstein's theory was difficult for many people to understand.

"D. Einstein's theory was difficult for many people to understand."