In the following passage, some of the words have been left out. Read the passage carefully and select the correct answer for the given blank out of the four alternatives.

With a fair possibility of liquid water being present on at least three planets, the focus is now (1)___________ deciphering the climate and chemical composition of their atmosphere. As a first measure, scientists are keen (2)___________ if the planets are Earth-like, by ruling (3)___________ the presence of hydrogen gas enveloping them. Mass estimates already suggest that the inner six planets (4)____________ have a rocky composition, while the one with a low density may have a volatile composition due to the (5)____________ of an ice layer or atmosphere.