What Combination of the Following Events Normally Prevents the Origins

Multiple Choice

What combination of the following events normally prevents the origins of replication in the yeast Saccharomyces cerevisiae from "firing" more than once during the cell cycle? Your answer is a two-letter string composed of letters A to E only, e.g. AB. Order the letters in your answer alphabetically.

A) The helicase-loading proteins Cdc6 and Cdt1 are only active in S phase.
B) The helicase Mcm1 can be delivered to the origin recognition complex (ORC) only in S phase.
C) The ORC can only become active (by dephosphorylation) in G? phase.
D) The helicase can only become active (by phosphorylation) in S phase.
E) The ORC can bind to the origin only in S phase.