Before dropping room prices from $100 to $90, the No-Tell Motel rented 3 rooms. Average total costs were $50 per room. After the price drop, it rents 4 rooms and average total costs increase to $60 per room. The motel's marginal costs of increasing output are

A) $240.

B) $150.

C) $50.

D) $60.

E) $90.