题目

    My friend Jennie Crossen gave her bone marrow (骨髓) to a 51-year-old woman with leukemia, a woman she has never met.     At the end of March, the Red Cross called back. Six different blood elements were tested; Jennie matched five completely and was about a 3/4 match of the sixth. So she was the best match.     In April, Jennie went for a complete physical exam by one of the doctors who would perform the surgery. The doctor had to make sure that it would be no threat to her. The doctor told her that the surgery would entail having two quarts of bone marrow extracted frHe tried his best to solve the problem, ______ difficult it was.A．howeverB．no matterC．whateverD．although