Twelfth Night: What convinces Sir Andrew that Olivia prefers him to Cesario?

A: The fact that she openly pursued Cesario to goad Sir Andrew
B: The fact that Cesario is too young to be any serious threat to him
C: The fact that she professed her love in a letter
D: The fact that Mario overheard Olivia admitting her love for Sir Andrew

C: The fact that she professed her love in a letter