Question

WHEN THE CHILDREN CAME BACK HOME, THEY WERE TIRED. THEY (TO SWIM) IN THE POOL ALL AFTERNOON

Answer 1

WHEN THE CHILDREN CAME BACK HOME, THEY WERE TIRED. THEY (had been swimming) IN THE POOL ALL AFTERNOON

Answer 2

WHEN THE CHILDREN CAME BACK HOME, THEY WERE TIRED. THEY HAD BEEN SWIMMING IN THE POOL ALL AFTERNOON.