24．       If nature does not provide man with the necessary material, it is the laboratory        he will turn to for it.

A．where                    B．which                    C．that                       D．what

25．       All flights        because of the snowstorm, they decided to take the train.

A．were cancelled                                          B．have been cancelled

C．had been cancelled                                     D．having been cancelled

26．       I really appreciate        to help me, but I am sure that I can manage it myself.

A．you to come                                             B．that you come

C．your coming                                             D．how you come

27．       A new system of quality control was        to overcome the shortcomings in the firm’s products.

A．invested                 B．informed                C．introduced              D．instructed

28．       It may be worthwhile at this moment to        and see what results we have got after one year’s experiments.

A．look back