The theory suggests that the interstellar dust within a nebular cloud is the most important component of planet formation.

The Nebular theory suggests that the interstellar dust within a nebular cloud is the most important component of planet formation. The Nebular theory or hypothesis is the most widely accepted among the scientists in regards to the formation of the solar system. 

Answer (d) may increase or decrease porosity through dissolution or deposition;

Answer (b) they're often rural migrants with no title to housing or rights to established businesses