On dividing 21 by a number, the quotient is 10 and the remainder is 1. Find the divisor.
Options:
A) 1 B) 2 C) 4 D) 6 E) 7

Rationale: "d = ( d - r ) / q = ( 21 - 1 ) / 10 = 20 / 10 = 2 b"
Answer: B