On an initially empty vector, what's the answer after executing: insert(0, 2), insert(1, 6), put(0, 1), remove(1), insert(0, 7) 在一个初始为空的向量上依次执行：insert(0, 2), insert(1, 6), put(0, 1), remove(1), insert(0, 7) 后的结果是：
A{1, 2, 7}
B{2, 1, 0, 7}
C{7, 1}
D{2, 1, 7}