Pipe A fills the pool in 4 hrs.

1 hour's work : 1/4

Pipe B empties the pool in 6 hrs.

1 hour's work : 1/6

Together if they work, 1 hour's work = 1/4 -1/6 = 1/12

Given : Pipe A started at 8:00 a.m and Pipe B at 9:00 a.m

Pool filled after 1 hour by Pipe A : 1/4 or 3/12

After 9:00 a.m

Pool filled after 1 hour with both the pipes on : 1/12

Pool filled after 9 hours with both pipes on : 9/12

Pool filled in 1 hour + Pool filled in 9 hours = 3/12 +9/12 =1

Therefore, it takes 10 hrs to fill the pool

As Pipe A started at 8:00 a.m, pool is full at 18:00 hrs

Answer is C.

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"I would risk the fall, just to know how it feels to FLY"