在双向链表尾部增加新节点的 append() 函数中，横线上应填写（ ）。
void append(int data) {
Node* newNode = new Node(data, nullptr, nullptr);
if (is_empty()) {
head = tail = newNode;
} else {
____________
}
++size;
}
- A. ```cpp
tail->next = newNode;
- **B.** ```cpp
newNode->prev = tail;
tail = newNode;
- C. ```cpp
tail = newNode;
newNode->prev = tail;
tail->next = newNode;
- **D.** ```cpp
tail->next = newNode;
newNode->prev = tail;
tail = newNode;