第二节：语法填空（共10小题；每小题1.5分, 满分15分）
If you walked past the supermarket, you would find many foods that have been treated ____16______ (kill) any harmful germs that might have been in them. Milk is a good example. Raw milk --- that is, milk just as it comes from the cow --- may be _____17_____ (safe) to drink. But ____18______ the milk is heated and then cooled, the harmful germs are killed. The man ____19______discovered this way of treating milk was a Frenchman by the name of Louis Pasteur.
In the supermarket you would also find many frozen foods --- frozen fruits, vegetables, meat and fish. As techniques for ____20______ (freeze) food are being improved, more frozen foods are appearing on the market, and more people are buying them. Fruits and vegetables to be sold in the supermarket are often frozen the moment ____21______are picked. The ____22_____(soon) fruits and vegetables are frozen, the better. _______23_____, machines are often taken into the fields where____24______food grows, so that little time is lost _____25_____picking and freezing.
，偏转极板间距为d，电子的电量为e。当电子加速后从两偏转极板的中央沿板平行方向进入偏转电场，电子的重力忽略不计。