In Which of the Following Scenarios Might an Offeror Revoke

In which of the following scenarios might an offeror revoke an offer of a unilateral contract without consequence? Please select all that apply.

A) At any point before the offeree has completed the requested act.
B) When the offeree has embarked upon the requested act but before any significant degree of progress has been achieved.
C) The offer expressly stated that the offeror was free to revoke it at any time and no benefit had been conferred on the offeror.
D) Before the offeree has begun to perform the requested act.