Which of the following is true of offer acceptance in a contract?

a$.$ In most cases, an offeree's silence and failure to act can be regarded as an acceptance.

b$.$ When an offeror has specified a particular manner and time of acceptance, generally, the offeree cannot accept in any other way.

c$.$ An offeror can frame an offer in such a way as to make the silence and inaction of the offeree operate as an acceptance.

d$.$ A party to an existing contract can effect a modification of that agreement without the other party's actual acceptance or approval.