Mahabharat questioned (Hindu mythology)

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. Kansa furious then ordered
to put Vasudev and Devki behind the bars. When the first son is
born, Kansa kills him by poisoning... second child is born and
Kansa throws him off the mountain peak...third child is born..."

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

Masterji: "Ramu bete, whole India does not have
doubt in mahabharata then how come you 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 ?

Masterji fainted.........................