Solaris, a Wisconsin-based telecommunications company, offers several residential phone service plans. Under their Volume Discount Plan, residential customers pay $.20 per minute for long distance calls and receive a 25% discount on their total long distance charges if the customer's monthly long distance charges meet or exceed $10.

Under this plan, the incremental cost of the 50th minute of phone service would be

A. $.20.

B. $.15.

C. -$2.50.

D. -$2.30.

E. Additional information is needed about the consumer's demand.