为便于判别有向图中是否存在回路，可借助于 ( )
A．广度优先搜索算
B．最小生成树算法
C．最短路径算
D．拓扑排序算法
参考答案：D
Miss Gao likes music. She spends a lot of money _____ CDs.[ ]
A. to buy
B. buying
C. buys
D. buy
In the family photo, my sister is ________ the left.
A．at
B．in
C．on
D．to
