Why can sharks swim quickly through water

The reason why sharks can swim fast is that unlike most fish, sharks have tails that are not horizontally symmetrical . Instead , the top of a shark's tail protrudes further  back than the bottom , giving the shark  lift while swimming and making it "spring" forward at speed.

Most fish make a single water "ring"  that moves backward at the end of their tail's stroke but sharks are thought to produce two rings because of their asymmetrical  tail which bespeaks a more efficient propelling action.

It has also been discovered that sharks can raise the tiny teeth-like structures called placoid scales on their skin to create tiny wells across the surface of their skin  which reduces drag just like the dimples on a golf ball, thus enabling the shark move forward faster.

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands

did you fail the test? xd