请教一道PREP1数学题

4.In a stack of boards at a lumber yard, the 20th board counting from the top of the stack is immediately below the 16th board counting from the bottom of the stack.  How many boards are in the stack?

(A) 38

(B) 36

(C) 35

(D) 34

(E) 32

正确答案是E, 我选的是D..