6. At present the number of bone marrow donors in mainland China is ____.
6． D。分析：利用“the number of bone marrow donors 和mainland”作为答案线索，这样发现答案相关句：(第3段)..compared fewer than 30,000 donors among mainland China’s 1.3 billion people, the newspaper said. (第4段) China needs a pool of at least 100,000 donors but testing them would cost..30，000/100，000 = 1/3， 因此判断D是答案。
 
A．about one percent of the total population
B．to be responsible for childhood leukemia
C   an urgent and tough task to be accomplished
E   an expensive cost to be paid
7. Some solvents and building materials are considered ____.
7． B。分析：consider 后通常跟to 或as ，因此判断B是答案。
 
A．about one percent of the total population
C   an urgent and tough task to be accomplished
E   an expensive cost to be paid
8. Obviously, recruiting voluntary bone marrow donors in mainland China is ___.
8． C。分析：根据搭配含义（很明显，中国的大陆招募骨髓移植自愿者是一项紧迫而艰巨的任务）判断C合适。