51.

1. crucial for
2. available in
3. resulting from
4. ascribed to

52.

1. in disguise of
2. in shades of
3. in search of
4. in place of

53.

1. This is how
2. That's why
3. It is not that
4. There exists

54.

1. paid
2. pays
3. pay
4. paying

55.

1. though
2. not to say
3. not just one
4. as well

56.

1. Until
2. While
3. Unless
4. Once

57.

1. as opposed to
2. in addition to
3. as a result of
4. in spite of

58.

1. benefits
2. defects
3. approaches
4. risks

59.

1. in case
2. in advance
3. in return
4. in particular

60.

1. effect
2. reason
3. outcome
4. conclusion

(Reference keys.)