If you are in any doubt as to you should be doing these exercises, consult your doctor.
A. that
B. which
C. what
D. whether
网友回答
A. that
B. which
C. what
(答案→)D. whether
解析：考查宾语从句。Whether引导宾语从句whether you should be doing these exercises;句意：对于你是否应该做这些实验，如果你有任何疑问，请咨询你的医生。故D正确。