If a - b = 4 and a2 + b2 = 18, find the value of ab.
Options:  A) 9  B) 1  C) 2  D) 18  E) 19

Rationale: "2 ab = ( a 2 + b 2 ) - ( a - b ) 2 = 18 - 16 = 2 ab = 1 . answer : b"
Answer: B