Doubt in Mahabharat

In some remote village of India, one masterji is teaching theMahabharat 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 bypoisoning... Second one is born n kansa throws him off the mountainpeak. 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 JAIL ???Masterji fainted.........................