An experimenter extracts and purifies act in filaments from ananimal cell. Using an appropriate visualization technique, herealizes that some actin filaments are very stable, somedepolymerize rapidly, and some depolymerize slowly. What could be aplausible reason for explaining the slow depolymerization of someof the filaments?

a) The filaments are capped at both ends.
b) The filaments are capped at the (+) end.
c) The filaments are capped at the (-) end.
d) The filaments are treadmilling.