51. I’ve kept up a friendship with a girl whom I was at school _____ twenty years ago.
A) about
B) since
C) till
D) with

52. He wasn’t asked to take on the chairmanship of the society, _____ insufficiently popular with all members.
A) being considered
B) considering
C) to be considered
D) having considered

53. _____ for the timely investment from the general public, our company would not be so thriving as it is.
A) Had it not been
B) Were it not
C) Be it not
D) Should it not be

54. The Social Security Retirement Program is made up of two trust funds,_____ could go penniless by next year.
A) the larger one
B) the larger of which
C) the largest one
D) the largest of which

55. Nowhere in nature is aluminum found free, owing to its always _____ with other elements, most commonly with oxygen.
A) combined
B) having combined
C) combine
D) being combined

51.D 52.A 53.A54.B 55.D