活塞压缩环的泵油现象主要是因为（）。
A．缸壁油量太多
B．压缩环装配不当
C．环磨损
D．压缩环天地间隙
参考答案：D
解析：
由于活塞环在气缸中要留有搭口间隙，因此正常工作的压缩环不可能完全阻断燃气的漏泄。再加上活塞环有可能出现的失效，为了提高密封效果，一个活塞上要设多道压缩环。活塞环在工作中由于天地间隙的存在，在轴向力的作用下会
产生上下运动。当活塞环向上运动时，环的下表面与环槽上表面间形成间隙，缸壁上油进入环槽中；当活塞环向下运动时，又把环槽中油挤到上方。这样随着柴油机工作循环的进行，滑油就被一道道环逐渐泵上去，最终泵入燃烧室，这种现象称为活塞环的泵油现象。根据活塞环泵油现象的机理可知，泵油现象主要是由于活塞环天地间隙的存在造成的。
| 阅读理解。 |
| Of all the fish we catch in the world, we eat only three quarters of it. The rest goes to glue (胶水) soap, margarine (人造奶油), pet food and fertillizer. Fishermen usually freeze fish they catch at sea. Back in port, they defrost the fish, make the fish have no bones in it and sell it as fresh fish. Over ninety-five percent of fish caught is in the northern hemisphere. Thus, only about five percent of all fish caught is from south of the equator. The Japanese are the world champion fish eaters. They eat twice as much fish as the Scandinavians, and five times as much fish as the Americans. The Russian sturgeon (鲟鱼) is the most expensive fish in the world. The eggs of the sturgeon are called caviar. |
| 1. What do we do with the fish we catch in the world? |
[ ] |
| Ａ: We eat all the fish we catch Ｂ: We use some of it to make pet food and fertilizer Ｃ: We do not use 25% of it Ｄ: We freeze all the fish we catch |
| 2. In the second paragraph, the word "defrost" means ____. |
[ ] |
| Ａ: make the fish dead Ｂ: make the fish alive Ｃ: make the fish become unfrozen Ｄ: make the fish clean |
| 3. Which of the following statements is correct according to the passage? |
[ ] |
| Ａ: Americans eat five times as much fish as the Scandinavians. Ｂ: Scandinavians eat five times as much fish than the Americans. Ｃ: Japamese eat more fish only than the Americans and Scandinavians. Ｄ: The Americans do not eat so much fish as the Japanese. |