sentence2_structure: Mercury<>is<>always<>closer to the sun than it is to earth$$$it<>is<>to earth
sentence1: The planet is compared to Mercury, Earth, Jupiter and Saturn, and 51 Peg is compared to the sun.
sentence2: Mercury is always closer to the sun than it is to earth.
question: Which of the following planets is always closer to the Sun than it is to Earth?