13. The health official said ____ 12 suspected cases of influenza A（H1N1）were tested negative and ___, so far, this city has reported no case of the virus.
A. what; that B. that; what C. 不填; that D. that; 不填
14. We went through a period ___ communications were very difficult in the rural areas.
A. which B. in which C. whose D. with which
15. It has been proved ___ eating vegetables in childhood helps to protect you against serious illnesses in later life.
A. if B. because C. when D. that
