One of the Reasons for the General Rule That Hearsay

One of the reasons for the general rule that hearsay evidence is inadmissible is:

A) the triers of fact can observe the demeanor of the declarant but not that of the person testifying.
B) the person testifying as to what he or she heard is not under oath.
C) the witness testifying as to what he or she heard cannot be cross-examined.
D) the out-of-court declarant is not under oath to speak the truth.