1£®When  _______ what is the most efficient way to protect oneself from COVID-19, the expert says no doubt it is wearing a mask.

A£®asking           B£®asked            C£®to ask           D£®having asked

2£®There is not much difference between the two, I really don't know _____.

A£®which should I choose                B£®what I should choose

C£®which I should choose                D£®what should I choose

3£®It was not until I got home _______ I found that I had left my keys in the office.

A£®before           B£®when             C£®then             D£®that