51.

1. duplicate
2. reproduce
3. mate
4. adopt

52.

1. degenerations
2. defeat
3. defects
4. deficiency

53.

1. was
2. were
3. has been
4. have been

54.

1. easier
2. more easy
3. difficult
4. more difficult

55.

1. on
2. into
3. about
4. out

56.

1. need to
2. used to
3. tend to
4. ought to

57.

1. each
2. one
3. right
4. equal

58.

1. unilaterally
2. unanimously
3. unnaturally
4. unintelligibly

59.

1. making
2. using
3. the sake of
4. the right of

60.

1. Male
2. Female
3. Both
4. Either

(Reference keys.)