The space agency believes that outstanding astronauts should have three conditions: first, rich knowledge; second, skilled technology; and third, strong will. Currently there are four A. B, C, and D who meet at least one of the conditions Excellent pilots sign up for the election. Known: ①A and B have the same level of strong will ②B and C have the same level of knowledge ③C and D are not all knowledgeable ④Three of the four are knowledgeable, two of them are strong and one is skilled The NASA's investigation revealed that only one of them fully met all the requirements of a good astronaut. he is? ['A.', 'B.', 'C.', 'Ding.']