Without the friction (摩擦力) between feet and the ground, people would ______ be able to walk.
We live too far from the city to get a strong TV ________.
It was last Saturday ________ we visited the History Museum.
I suggested that the problem _____as soon as possible.
When ________ to the market, these products were very popular.
His English, ______ used to be very poor is now excellent.