Adouble-displacement reaction occurs when the negative ion of me compound replaces the positive ion of the other compound to form two new compounds during a chemical reaction.

a. true
b. false

B. False

Explanation: A double displacement reaction is one in which exchange of ions take place. The salts which are soluble in water are designated by symbol (aq) and those which are insoluble in water and remain in solid form are represented by (s) after their chemical formulas.

Example:

Thus as we can see sodium ion is being replaced by ammonium ions   and thus positive ions replace positive ions only and not negative ions.

i believe it is-

d.) gravity pulled particles of dust and gas together to form planets.

a  solution  is always transparent, light passes through with no scattering from solute particles which are molecule in size. the  solution  is homogeneous and does not settle out. a colloid is intermediate between asolution  and a  suspension. while asuspension  will separate out a colloid will not.