A steady decline in annual movie-ticket sales is about to begin. More than half of the tickets sold last year were sold to the age group under twenty-five years of age, representing twenty-seven percent of the population. However, the number of individuals under twenty-five will steadily decline during the next decade.
Which of the following, if true, casts most doubt on the prediction above regarding future movie-ticket sales

A．Medical advances have lowered the mortality rates for those who are forty to sixty years of age.

B．Many people gradually lose interest in going to the movies after they reach twenty-five years of age.

C．The number of movie theaters has been increasing, and this trend is expected to continue during the next ten years.

D．Movie-ticket sales tend to increase as the size of the work force increases, and the size of the work force will increase annually during the next decade.

E．(E) Experts agree that people under twenty-five years of age will continue to account for more than half of the total number of tickets sold in each of the next ten years.

如图所示是重机主钩的控制原理图，现该主钩电机往上升方向运行正常，往下降方向不动作，对照控制原理图分析其故障原因有哪些？