4. As he usually hid his money in different shelves, sometimes he just could not remember _____ shelf he had kept it.
A. on which B. what C. in which D. when
5. The foreign employer just could not understand _____ so many of his employees had given up their higher pay to work for other companies.
A. whether B. that C. why D. when
6. It was kind of you to inform us of _____ you had known about Peter. You know we had not heard from him for more than a year.
A. when B. if C. how D. what
7. The new law will come into effect on November 1 _____ no children under 18 will be allowed to buy either wine or cigarettes.
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