The passage is mainly concerned with _________.

Câu hỏi: Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the following questions.
Fish that live on the sea bottom benefit by being flat and hugging the contours. There are two very differcent types of flatfish and they have evolved in very separate ways. The skates ad rays, relatives of the rharks, have become flat in what might be called the obivious way. Their bodies have grown out sideways to form grat "wings". They look as though they have been flattened but have remained symmetrical and "the right way up". Conversely, fish such sas plaice, sole, and halibut have become flat in a different way. There are bony fish which have a marked tendency to be flattened in a vertical direction; they are much "taller" than they are wide. They use their whole, vertically flattened bodies as swimming surfaces, which undulat through the water as they move. Therefore, when their ancestors migrate to the seabed, they lay on one side than on their bellies. However, this raises the problem that one eye was always looking down into the sand and was effectively useless – In evolution this problem was soleved by the lower eye "moving" around the other side. We see this process off moving around enacted in the development of every young bony flatfish. It starts life swimming near the surface, and is symmetrical and vertically flattened, but then the skull starts to grow in a strange asymmetrical twisted fashion,, so that one eye, for instance the left, moves over the top of the head upwards, an old Picasso – like vision. Incidentally, some species of 20 flatfish settle on the right side, others on left, and others on either side.
The passage is mainly concerned with _________.
A. symmetrical flatfish
B. bony flatfish
C. evolution of flatfish
D. different types of flatfish