Doubt In Mahabharat !!

Coolest doubt in MahabharatIn some remote village of India, one masterji is teaching the MahabharatKatha to class 6 students. He is at the krishnajanma' part of it.Masterji: "Kansa heard the akashwani that his sister's 8th child isgoing to kill him. He was furious. He ordered to put vasudev n devkibehind the bars.First son is born, and kansa kills him by poisoning... Second one isborn 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 havea doubt(sounding nervous n confused)Masterji: "Ramu bete, whole India does not have doubt in MahabharataThen how come u have one?"Ramu : Masterji, if Kansa knew that Devaki's 8th child was going to killhim,WHY THE HELL DID HE PUT VASUDEV AND DEVAKI IN THE SAME CELL ?Masterji fainted.........................