Why Are the Ends of Microfilaments and Microtubules Called "Plus

Why are the ends of microfilaments and microtubules called "plus ends" and "minus ends"?

A) Because monomers are added more quickly to one end (the "plus end") than they are to the other end (the "minus end") .
B) Because polymerization occurs at one end (the "plus end") and depolymerization occurs at the other end (the "minus end") .
C) Because monomers can be added to one end only (the "plus end") , and can only be removed from the other end (the "minus end") .
D) Because the actin and tubulin monomers that make up microfilaments and microtubules have evolved more rapidly than most other proteins.