51. In the meantime, the question facing business is whether such research is _____ the costs.

A. worth

B. worth of

C. worthy

D. worthwhile

52. In the preface _____ the book, the old scholar attaches great importance to practicalness.

A. to

B. for

C. about

D. of

53. In this experiment, they are wakened several times during the night, and asked to report what they _____.

A. had just been dreaming

B. have just been dreaming

C. are just dreaming

D. had just dreamt

54. In this factory the machines are not regulated ______ but are jointly controlled by a central computer system.

A. independently

B. individually

C. irrespectively

D. irregularly

55. It _____ five years since the famous singer left her hometown.

A. was

B. is

C. has passed

D. had been