Alex Watches His Mother Play Tennis in the Morning

Alex watches his mother play tennis in the morning.Later,when he is alone,Alex picks up a stick and swings it like a tennis racquet,copying his mother's actions from earlier in the day.According to Piaget,Alex is in the _____ substage of sensorimotor development.

A) 3^rd
B) 4^th
C) 5^th
D) 6^th