3. What keeps Earth revolving around the sun? A. the force of gravity
B. the pull of the moon
C. the tilt of Earth's axis
D. the rotation of Earth

A. The force of gravity.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

a corroding iron, if that's one of your choices