Which of the Following Details the Nucleotide-Binding Site When Tubulin $\alpha$

Which of the following details the nucleotide-binding site when tubulin dimers assemble into microtubules?

A) both $\alpha$ and $\beta$ sites are bound to GTP
B) the $\alpha$ site is bound to GTP; the $\beta$ site is bound to GDP
C) the $\alpha$ site is bound to GDP; the $\beta$ site is bound to GTP
D) both $\alpha$ and $\beta$ sites are bound to GDP
E) the $\alpha$ site is bound to GTP; the $\beta$ site is unoccupied following GTP hydrolysis and release of GDP