_____ your sister _____ playing the piano these days?

The physics teacher said that light _______ much faster than sound.

—Are you going to help John with his Chinese this evening?—No，He ________ to England．He will be back next month．

—Have you mended your shoes, Bob？—Yes, I ____ them twenty minutes ago ．

—How well do you know the Opera House?—I know the place very well. I ____ Sydney many times.