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);