小明自己制作了一个哨子，如图所示．在筷子上缠一些棉花，做成一个活塞，用水蘸湿棉花后插入两端开口的塑料管．吹管的上端，可以发出悦耳的哨声．这哨声是由管内空气柱 ____产生的．上下推拉活塞，可以改变声音的 ____（选填“音调”，“响度”，或“音色”）．

​

⑴____ A. vacation B. exam C. David D. favorite

⑵____ A. most B. post C. host D. cost

⑶____ A. seem B. feel C. coffee D. between

⑷____ A. however B. blow C. brown D. now

⑸____ A. clearly B. hear C. learn D. near