A defendant was charged with murdering his boss. After obtaining a valid search warrant and executing a valid search of the defendant's office, an officer found a love letter from the defen­ dant's wife to his boss describing their sexual relations. The letter stated, "I can no longer hide my love for you from my husband. I intend to tell him about us and leave him for you." At trial, the officer seeks to testify about the contents of the letter as proof of the defendant's motive for killing his boss. The defense counsel should object on which of the following grounds?
选项：

A:The letter was not authenticated as being from the defendant's wife.
B:The testimony violates the best evidence rule.
C:The testimony is hearsay not within any exception.
D:The probative value of the evidence is substantially outweighed by the danger of unfair prejudice.