What does "it" in paragraph2, sentence 3 refer to

A．There are many planets which are like the earth.
B．We have been sending radio messages to the outer space since the beginning of the last century.
C．The distance between us and other inhabitants of the universe is perhaps too large to be conquered.
D．There are probably other intelligent life like us existing somewhere in the universe.

A．Sooner or later, we can find intelligent life on stars.
B．Many planets have the same condition as our earth.
C．Life depends on the stars like the Sun.
D．We will certainly receive the messages sent to us by other intelligent beings.