13 對兩個空堆疊（empty stacks）S1 及 S2 分別執行下列步驟後，何者的最後一個 S2.pop( )之回傳值為 B？（註：push 為加入一元素到 stack 之動作，pop 為由 stack 取出一元素之動作。）(A) S1.push, S1.push, S2.push, S1.push(S2.pop( )), S2.push(S1.pop( )), S2.pop( ) (B) S1.push, S1.push, S2.push(S1.pop( )), S2.push, S1.push(S2.pop( )), S2.pop( ) (C) S1.push, S2.push(S1.pop( )), S1.push, S2.push, S1.push(S2.pop( )), S2.pop( )(D) S2.push, S1.push(S2.pop( )), S1.push, S2.push(S1.pop( )), S1.push, S2.pop( )