A person standing on the escalator takes time t1to reach the top of a tower when the escalator is moving. He takes time t2 to reach the top of the tower when the escalator is standing. How long will he take if he walks up a moving escalator?
(A) t2−t1
(B) t1+t2
(C) t1t2/t1−t2
(D) t1t2/t1+t2