Doubt in Mahabhart !

In some remote village of India, 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 and 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' N' DEVAKI ' IN THE SAME JAIL ??? Masterji fainted.........................