Whether a Microtubule Shrinks or Grows Is Determined by the $\beta$

Multiple Choice

Whether a microtubule shrinks or grows is determined by the

A) rate of GTP-bound tubulin addition relative to the rate of tubulin GTP hydrolysis.
B) phosphorylation state of $\beta$ -tubulin.
C) rate of ATP hydrolysis relative to the rate of ATP-bound tubulin addition.
D) presence or absence of $\gamma$ -tubulin.