In a certain brick wall, each row of bricks above the bottom row contains one less brick than the row just below it. If there are 6 rows in all and a total of 135 bricks in the wall, how many bricks does the bottom row contain ?  Choose the correct choice from the following answers
A) 21
B) 20
C) 22
D) 19
E) 24

Rationale: "the bottom row has x bricks x + x - 1 + x - 2 + x - 3 + x - 4 + x - 5 = 135 6 x - 15 = 135 6 x = 120 x = 20 answer : b"
Answer: B