Question 12: I think ________ outside in the open air is much more enj

Question 12: I think ________ outside in the open air is much more enjoyable than ________ exercise in some stuffy gyms.

• A. to walk – done
• B. walking – doing
• C. walk – to be doing
• D. having walked – do

V-ing đóng vai trò là danh từ làm chủ ngữ, 2 vế song song nên đều phải để V-ing