鼻塞给氧或鼻导管给氧的缺点不包括（）。

A.吸氧浓度不恒定

B.对局部有刺激

C.易于堵塞

D.不适用于慢性阻塞性肺疾病（COPD.患者

E.提供的吸氧浓度不能超过50%

One of the most convenient and cheapest ways to see America is by riding a Greyhound bus. This interstate bus system connects all major cities in the United States, (52) people with frequent and convenient service. The bus system even has an international service (53) makes connection with cities in Canada and Mexico.

Its network even extends to some of the smaller towns and out-of-the-way communities (54) the great interior of the country. Traveling by bus may (55) longer than flying by plane, but the terminals are located in the center of most cities and there is (56) to the downtown area. These buses are comfortable and air-conditioned. They are all equipped with toilets in the rear to (57) the convenience of the passengers, but there are some very severe (58) of conduct which are strictly enforced. On all buses (59) is forbidden and the consumption of alcoholic drinks is not allowed.

(60) bus travel may not be suited to everyone’s taste, it affords budget travelers the (61) to see America in comfort and safety and at a leisurely unhurried pace.

54().

A.for 

B.along 

C.in 

D.from