如图，拿一张硬纸片，用同样的力在木梳上划过，如图所示，一次划得快些，一次划得慢些，比较一下当快划时，木梳齿振动得____ 些(选填“快”或“慢”)，听到的声音____ 些(选填“低”或“高”)；当慢划时，木梳齿的振动____ 些(选填“快”或“慢”)，听到的声音____ 些(选填“低”或“高”)．

清朝初期规定了西藏历世达赖和班禅都必须经过中央政权册封的制度。此项制度的主要作用在于（    ）

    The US is the fourth largest country in the world. It's smaller than Russia and China. Washington DC is its capital. But it is a quiet city.New York is the biggest city in America and it's one of the biggest cities in the world. It is the east of the US and at the south of the Hudson River(哈德逊河).A large part of the city is on Manhattan Island(曼哈顿岛), a big island in the river. New York has a larger population(人口) than Washington DC. In New York, we can see a famous statue, the Statue of Liberty(自由女神像)

⑴⑵⑶根据短文内容，选择正确的单词或短语把句子补充完整；⑷⑸根据短文内容，选择正确答案。

我国导弹部队担负的主要任务是（    ）