In which sentence is the past participle used incorrectly?
a. the nursery school class had had music time this morning for an hour.
b. the town was in the middle of a choir competition, and those that had performed already had sang beautifully.
c. the gardener had lain her tools down by the shed and promptly forgotten them.
d. mike began to keep his glasses on a cord as he could never remember where he had put them.