A young man was going to join the army (军队

问题：

A young man was going to join the army (军队) and had to (41) a medical examination. The doctor was sitting at a desk when he came in. He said to the man, "Take your coat and shirt off and (42) on that chair.
The young man did so. The doctor looked at him for a moment without (43) up from his chair and then said, "All right. Put your clothes on again.
"But you haven’t examined me at ail!" the young man said in (44) .
"There is no need to do so," the doctor said with a (45) .
"When I (46) you to take your coat and shirt off, you heard me all right, so there is nothing wrong with your (47) . You saw the chair, so your eyes are (48) . You could take your clothes off and sit on the chair, so you must be in good health. And you understand what I told you to do and did it without a (49) , so you (50) healthy enough for the army.\

[A] can be [B] must[C] must be