For a solar eclipse to occur,
a. the sun must be directly between earth and the moon.
b. the moon must be directly between earth and the sun.
c. the moon must be directly behind earth.
d. earth must be directly between the sun and the moon.

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.