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 child
was going to

kill

him,

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