In the xy-coordinate system, if (m, n) and (m + 4, n + k) are two points on the line with the equation x = 2y + 5, then k =  Pick one
A) 1 / 2 B) 1 C) 2 D) 5 / 2 E) 4

Rationale: "since ( m , n ) and ( m + 2 , n + k ) are two points on the line with the equation x = 2 y + 5 they should satisfy m = 2 n + 5 and m + 4 = 2 * ( n + k ) + 5 . by 1 st equation we have m - 2 n = 5 and by 2 nd equation m - 2 n = 2 k + 1 - - - > 5 = 2 k + 1 - - - > k = 2 . the answer is , therefore , ( c ) ."
Answer: C