题目

 如图为回旋加速器的示意图．其核心部分是两个D型金属盒，置于磁感应强度大小恒定的匀强磁场中，并与高频交流电源相连．带电粒子在D型盒中心附近由静止释放，忽略带电粒子在电场中的加速时间，不考虑相对论效应．欲使粒子在D型盒内运动的时间增大为原来的2倍，下列措施可行的是(   ) A. 仅将磁感应强度变为原来的2倍        B. 仅将交流电源的电压变为原来的1/2 C. 仅将D型盒的半径变为原来的倍     D. 仅将交流电源的周期变为原来的2听1段材料，回答1-5小题。1. What was the weather like? [     ]A. Sunny and warm. B. Sunny but cold. C. Rainy but warm.2. What was the old man holding in his hand? [     ]A. A big black umbrella. B. A big strong walking stick.C. A strong black walking stick.3. Why did the old man carry such an umbrella? [     ]A. Because he wanted to keep the sun off. B. Because he wanted to keep the rain off. C. Because he wanted to use the umbrella as a walking stick.4. What was wrong with the old man? [     ]A. He broke his leg badly. B. His legs were not strong enough. C. He was afraid of seeing the sunshine. 5. What was the old man like in the passage? [     ]A. Clever.B. Foolish. C. Dishonest.