Coolest doubt in Mahabharat!!!

In some remote village of India, one masterji is teaching the Mahabharat katha to class 6 students. He is at the krishnajanma' part of it.

Masterji:"Kansa heard the akashwani that his sister's 8th child is going to kill him.He was furious. He ordered to put vasudev n devki behind the bars. First son is born, and kansa kills him by poisoning. Second one is born n kansa throws him off the mountain peak. Third one is born."

Now Ramu, who is smartest of the lot, puts up his hand. Masterji, I have a doubt (sounding nervous n confused)

Masterji: "Ramu bete, whole India does not have doubt in Mahabharata then how come u have one?"

Ramu : Masterji, if Kansa knew that Devaki's 8th child was going to kill him,

WHY THE HELL DID HE PUT VASUDEV AND DEVAKI IN THE SAME CELL ??