题目

One day a teacher stood before his class and picked up a large empty glass jar and filled it with rocks. He then asked the students if the jar was_________. They agreed that it was. He then added a box of pebbles(鹅卵石)into the jar,_________it lightly. The pebbles rolled into the open areas between the rocks. “Is the jar full now?” The teacher asked. The students said yes.But then the teacher poured a bag of sand into the jar. Once more he asked _________ it was full. The students_________said that it was.The teacher then poured a glass of beer_______the jar. The students were very exc请回答下列问题：⑴甲图是研究短路引起熔丝熔断的实验，则应该将导线接在_________（选填AC或BD）两点间造成短路，引起熔丝熔断。⑵乙图是为了研究温度对导体电阻的影响，设计实验电路：用酒精灯对电路中的一段灯丝（阻值约30Ω）加热，观察导体电阻的变化，该实验方案存在缺陷是_________________，改进方案：___________________。⑶丙图，在“探究通电螺线管外部磁场”的实验中，在有机玻璃板上均匀的撒满铁屑，并在铁屑上放几个小磁针，放个小磁针目的是__________________，闭合开关后为了更好地通过铁屑客观地描述磁场分布情况，接下来的操作是_______________，操作的主要目的是减小铁屑与玻璃板之间的摩擦，使铁屑在磁场力的作用下动起来表明通电螺线管周围有磁场。