There are three requirements for recruiting employees from a certain unit: first, knowledgeable; second, skilled; third, hardships and hardships. There are four job applicants A. B, C, and D who meet at least one of the conditions. Knowing: ① A and B are not both knowledge-rich; ② A and D have the same level of knowledge; ③C and D have the same level of hardship and hard work; ④ Three of the four are knowledgeable, two hard and hard, and one is skilled. The unit was assessed and found that only one of them met all the conditions. Should it be? ['A.', 'B.', 'C.', 'Ding.']