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.