Which of the following describes a mechanism by which microtubules are known to be statſilized? a) Shortening of the GTP cap. b) Increasing the proportion of alpha-tubulin subunits relative to beta-tubulin subunits. c) Binding of microtubule binding proteins, such as Tau. d) Conformational changes undertaken by beta-tubulin as it bhydrolyzes GTP.