单项选择题
How many more decades will have to pass （） scientists succeed in providing a cure for cancer？
A．when B．before C．since D．until
单项选择题 The disease beri-beri （）．
单项选择题 According to the author, many teenagers think they are brave enough to act on their own, but in fact most of them （）．
单项选择题 According to Paragraph 2, without computers （）．
