下面函数使用快慢指针查找单链表的中间结点。横线处应填写（　　）。

struct Node {
    int value;
    Node *next;
};

Node *middle(Node *head) {
    Node *slow = head;
    Node *fast = head;
    while (fast != nullptr && fast->next != nullptr) {
        slow = slow->next;
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    }
    return slow;
}

{{ select(1) }}

• fast = fast->next;

• fast = fast->next->next;

• fast = slow->next;

• fast = head->next;