Which of the Following Statements About the Structure of Microtubules

Which of the following statements about the structure of microtubules is FALSE?

A) Microtubules are built from protofilaments that come together to make a hollow structure.
B) The two ends of a protofilament are chemically distinct, with α-tubulin exposed at one end and β-tubulin exposed at the other end.
C) Within a microtubule, all protofilaments are arranged in the same orientation, giving the microtubule structural polarity.
D) α-Tubulin and β-tubulin are covalently bound to make the tubulin dimer that then assembles into protofilaments.