1£®—How long have you been here, Susan?

—Only about five minutes. Alisa ________ (walk) here with me.

2£®One third of the country ________ (cover) with trees.

3£®—You've made great progress in your English studies, haven't you?

—Yes, but much ________ (remain) to be done.

4£®It's noticeable that the air quality in cities ________ (be) becoming worse and worse recently.