A man can row his boat with the stream at 18 km/h and against the stream in 4 km/h. The man's rate is?
Choose the correct answer.
A) 1 kmph  B) 3 kmph  C) 8 kmph  D) 7 kmph  E) 5 kmph

Rationale: "ds = 18 us = 4 s = ? s = ( 18 - 4 ) / 2 = 7 kmph answer : d"
Answer: D