A man can swim in still water at 9 km/h, but takes twice as long to swim upstream than downstream. The speed of the stream is?  Options:
A) 1.7
B) 1.9
C) 3
D) 1.5
E) 1.2

Rationale: "m = 9 s = x ds = 9 + x us = 9 - x 9 + x = ( 9 - x ) 2 9 + x = 18 - 2 x 3 x = 9 x = 3 answer : c"
Answer: C