The Current Model for the Nucleation of Microtubules Is That $\gamma$

Multiple Choice

The current model for the nucleation of microtubules is that a helical array of $\gamma$ -tubulin subunits forms an open,lock-washer-like ring-shaped template upon which the first row of $\alpha$$\beta$ -tubulin dimers assembles.How does this model account for the polarity of microtubules?

A) Only the $\alpha$ -tubulin of a heterodimer can bind to the ring of $\gamma$ -subunits.
B) Only the $\beta$ -tubulin of a heterodimer can bind to the ring of $\gamma$ -subunits.
C) The ring structure straightens out the microtubule.
D) The ring structure interacts with a homodimer instead of a heterodimer.
E) Only the $\alpha$ -tubulin of a heterodimer can bind to the ring of $\gamma$ -subunits and the ring structure straightens out the microtubule.