51.

1. suggests
2. suggesting
3. having suggested
4. suggested

52.

1. since
2. although
3. whereas
4. when

53.

1. what
2. whom
3. who
4. as

54.

1. predicted
2. notorious
3. proven
4. detailed

55.

1. where
2. when
3. if
4. whatever

56.

1. as many
2. as much
3. as well
4. as often

57.

1. threat
2. puzzle
3. obstacle
4. dilemma

58.

1. detection
2. deletion
3. amplification
4. addition

59.

1. Therefore
2. Furthermore
3. Nevertheless
4. Conclusively

60.

1. likelihood
2. function
3. influence
4. flexibility

(Reference keys.)