Work in pairs. Explain the difference between sentences a and b in each pair. (Làm việc theo cặp. Giải thích sự khác nhau giữa câu a và b trong mỗi cặp) 1. a I'm doing my homework after school. b I do my homework after school. 2. a Joe is learning to drive. b Many teenagers learn to drive when they are seventeen. 3. a Mark plays the guitar. b Mark is playing the guitar.