Research on Motor Development Using Gesell's Co-Twin Method Found That

Research on motor development using Gesell's co-twin method found that

A) the trained twin learned to climb stairs earlier than the untrained twin.
B) the control twin learned to climb stairs earlier than the trained twin.
C) the trained twin and the untrained twin learned to climb the stairs at the same time.
D) training was effective, but only if it was longer than 3 weeks.