After using reagents in chemical lab, you should to（）
A. put it back in place immediatel
B. put it in an easy accessing place
C. make up by yourself,when it is insufficient
D. keep the bottle open for easy use next time
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列出SC表中所有考试不及格的学生的学号Sno（无须重复），其SQL语句为（）
A. SELECT DISTINCT Sno FROM SC WHERE Grade＜60;
B. SELECT DISTINCT Sno FROM SC WHERE Grade IS NULL;
C. SELECT Sno FROM SC WHERE Grade＜60;
D. SELECT ALL Sno FROM SC WHERE Grade＜60;
Do not wear（）in inflammable and explosive places
A. Cloth shoes
B. Rubber shoes
C. Spiked shoes
D. Sneakers
which is bad habit for use of laboratory refrigerator（）
A. Regular defrosting and cleaning
B. Disinfect the inner surface regularly
C. All chemicals stored shall be marked with the name , the date and the storer
D. The flammable solution placed in the ordinary refrigerator shall be noted on the door of refrigerator
The instantaneous center of velocity of two links that form a higher pair is located__（）
A. On the common normal line of the contact point
B. On the common tangent line of the contact point
C. At the contact point of the two links
D. Infinity point along the common normal line of the contact point
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