A horse is tethered to one corner of a rectangular grassy field 36 m by 20 m with a rope 18 m long. Over how much area of the field can it graze?
Choose the correct choice from the following answers.
A) 154 cm 2
B) 308 m 2
C) 254 m 2
D) 260 m 2
E) none of these

Rationale: "area of the shaded portion = 1 ⁄ 4 × π × ( 18 ) 2 = 254 m 2 answer c"
Answer: C