Which list correctly describes the usual order of planets inward toward the sun?
a. mars, earth venus, mercury, neptune, uranus, saturn. jupiter
b. mer, v,e, mars, j,saturn, u,nept
c. j, saturn, u,nept, mars, e,v, mer
d. n, u,sat, j,mars, e,v, mer

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.