Альтернативный вопрос - вопрос с выбором между чем-либо

После тире написан краткий ответ

a) Was he typing letters or answering the phone when the light went out? - answering the phone

b) Was Carol crossing the road or waiting for the bus when she saw the accident? - crossing the road

c) Were you talking on the telephone or watching TV when the door bell rang? - watching TV

d) Was it raining or snowing all day yesterday? - snowing