∵AP is the tangent of circle O, and P is the tangent point.
∴∠OPA=90,
∴∠O=90 -∠A=50,
OB = OP,
∴∠OPB=∠OBP=( 180 -∠O)÷2=65,
∴∠APB=90 -∠OPB=25。
So choose a.