20. 下列程式碼片段何者執行雙向鏈結串列的節點刪除（this 指向目前節點，pre 及 next 分別指向 this 之前、後節點）？
(A) this-＞next-＞pre = this-＞next;this-＞pre-＞next = this-＞pre;free(this);
(B) this-＞pre-＞next = this-＞pre;this-＞next-＞pre = this-＞next;free(this) ;
(C) this-＞pre-＞pre = this-＞next;this-＞next-＞next = this-＞pre;free(this);
(D) this-＞pre-＞next = this-＞next;this-＞next-＞pre = this-＞pre;free(this);