In the fifth paragraph, the underlined word "it" refers to ______.

A．Scientists know the answer to the question how a pigeon tells direction.
B．Scientists are trying to answer the question how a pigeon finds its way home.
C．A pigeon can use some special way to sense where its home is.
D．By using the sun a pigeon can tell directions, rain or shine.

A．they have a built-in compass
B．they can cover 500 miles in a single day
C．they have special ability to tell directions
D．they can find their way home