Which sentence is a simple sentence?

Responses
A When the monster arrived, the children screamed.When the monster arrived, the children screamed.
B When the children heard strange sounds, they hid under the bed.When the children heard strange sounds, they hid under the bed.
C The enormous, hairy monster with sharp teeth growled at the young children.The enormous, hairy monster with sharp teeth growled at the young children.
D The children will not return to the forest after the monster attacked Calvin.

D The children will not return to the forest after the monster attacked Calvin.