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两道SAT语法题sugar alcohols ---- sugar substitutes used in variou

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两道SAT语法题
sugar alcohols ---- sugar substitutes used in various brands of sugar free chewing gum --- do not cause tooth (decay but do contain calories)
这里不能用decay; although they contain calories 是不是因为分号后面要跟主语而不能跟连接词
increased competition in this market will certainly lower prices (but some economists warn) that it may also lower quality of service
请问这里为什么不能用however,some economists have warned
第一个although和but的位置问题,although位于句首,but位于第二个分句开始;
第二个however不能直接连接两个分句,必须另起新句,并用逗号隔开.例如:It was raining hard.However,we went out to look for the boy.
再问: 用分号隔开意味着开始新的句子所以不能用although开始? 而but是连接两个句子所以可以是吗....
再答: 举个例子,Although he was tired, he kept on working.这是对的,He was tired,but he kept on working.although和but在连接的两个句子中的位置是不一样的,although一定在前一个分句,而but在后一个分句。