春秋时期，农业生产水平提高的重要标志是（ ）
A：铁制农具和牛耕的使用
B：农业技术的进步
C：水利工程的兴修
D：农业产量的提高
— It's dangerous to swim in this river.
—Yes, you are right. The government has ______ people not to swim in it.
Can you hear Lucy_____ loudly in the next room?
下列物质是手性分子的是（ ）
