Many students get a driver's permit at 15. In California, they are then allowed to drive for 50 hours under supervision. “Under supervision” means that a friend or relative who already has a license has to ride with them. But what happens when teenagers (persons between 13 and 19 years old) start driving on their own? Many are careful drivers and never have a problem. But many others are not, and they create(造成) problems on the road. Fifteen-year-olds are too young to drive, and sixteen-year-olds are too immature(不成熟的) to start driving alone. In the United States, more teenagers are drilled in driving accidents than in any other way. While fewer than 7% of all drivers are teenagers, teenagers are involved(占；包括) in 14% of the terrible car accidents in the country. Sixteen-year-olds are involved in over 43 crashes in every million kilometers they drive. But seventeen-year-olds cause only about 30 accidents in a million kilometers of driving. One year makes a difference. So the age limit(限制) should be increased.

“团结就是力量”，下列最能说明这个道理的是（     ）

在$△ABC$中，若$\sin A\sin B={\cos }^2\frac{C}{2}$，则$△ABC$是（   ）

印度主要的粮食作物是