题目

Directions: Read the following text and choose the most suitable heading from A-F for each paragraph．There is one extra heading which you do not need. A．Causes of overusing fluorideB．Negative effects of fluoride C．Safe use of fluorideD．The expert’s background information E．No need to worry about fluoride poisoning F．Solutions to improper use of fluoride I have heard that fluoride can help stop dental decay, but I recently read that it is poisonous．Which of these is true? How can children use fluoride safely? This question is answered by Dr Marcia M．Rich who practises gen均匀直角金属杆aob可绕水平光滑轴o在竖直平面内转动，oa＜ob。现加一水平方向的匀强磁场B并通以电流I，若撤去外力后恰能使直角金属杆ob部分保持水平，如图所示．则                                  ［       ］A．电流I一定从a点流入才能使杆保持平衡．B．电流I从b点流入也可能使杆保持平衡．C．直角金属杆受到的顺时针力矩一共有二个．D．直角金属杆受到安培力与重力的合力为零．