题目

Bananas are one of the world’s most important food crops.They are also one of the most valuable exports.Bananas do not grow from seeds.Instead，they grow from existing plants.Bananas are threatened by disease because all the plants on a farm are copies of each other.They all share the same genetic weaknesses.For example，the Cavendish banana is most popular in North American and European markets.However，some kinds of fungus organisms easily infect the Cavendish.Black Sigatoka disease affects the leaves of Cavendish banana plants.The disease is controlled on large farms by putting 如图是法拉第研制成的世界上第一台发电机模型的原理图．将铜盘放在磁场中，让磁感线垂直穿过铜盘，图中a、b导线与铜盘的中轴线处在同一平面内，转动铜盘，就可以使闭合电路获得电流．若图中铜盘半径为L，匀强磁场的磁感应强度为B，回路总电阻为R，从上往下看逆时针匀速转动铜盘的角速度为ω．则下列说法正确的是（ ）A．回路中有大小和方向作周期性变化的电流B．回路中电流大小恒定C．回路中电流方向不变，且从b导线流进灯泡，再从a流向旋转的铜盘D．若将匀强磁场改为仍然垂直穿过铜盘的正弦变化的磁场，不转动铜盘，灯泡中也会有电流流过