The seashore was a barrier for descendants of fish-type

Đáp án A
Giải thích: Thông tin nằm ở: “These vertebrates, the fish, were adapted to underwater living. Their streamlined bodies were covered with scales to reduce surface friction: they had muscular tails so that they could swim swiftly in such a dense medium as water; and they were endowed with gills for breathing underwater. Descendants of fish-type ancestors crossed the seashore barrier and accommodated themselves to life on land.”