51.   Congress has on several occasions given extensive consideration to the problem of protecting the military and strategic secrets of the United States.

52.   Consider the problems of traffic accidents more closely.

53.   Critics say the Internet has amplified the problem of medical misinformation.

54.   Despite this, the problems of retirement should be kept in perspective.

55.   Doors that swing inwards present the problem of curtain fabric interfering with the operation of the doors.

56.   During the eighteenth and nineteenth centuries many studies were undertaken in hopes of solving the difficult problems of animal locomotion.