Mahabharath - Great Doubt

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)

"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 IN JAIL ?

Masterji fainted.........................no answer.....