What is the average of first 21 multiples of 7 ?  Select the correct option: A) 67 B) 69 C) 77 D) 79 E) 81

Rationale: "required average = 7 ( 1 + 2 + . . . . + 21 ) / 21 ( 7 / 21 ) x ( ( 21 x 22 ) / 2 ) ( because sum of first 21 natural numbers ) = 77 c"
Answer: C