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