31.   Other agents have been charged with helping drug smugglers and passing guarded economic secrets to private businesses.

32.   Once a closely guarded military secret, this composite, nonmetallic substance makes the aircraft almost invisible to radar.

33.   Precisely how lenticular images are prepared is a closely guarded secret.

34.   Since I am in the pre-IPO stage, the exact details of my company are still a closely guarded secret.

35.   So such information is a closely guarded secret, though from time to time there are slips and leakages.

36.   Some families maintained contact through letters and telephone calls, but kept their leper relatives a closely guarded secret.

37.   Staff, faculty and students were stunned by the revelations, made public in May, which had been a closely guarded secret at the university.

38.   Technical details of the security system were supposed to be a closely guarded secret, known only to a tight circle of industry engineers.

39.   The actual figure is a closely guarded secret.

40.   The actual recipe is a heavily guarded secret.