题目

词汇运用         Most people   1  (喜欢) to eat sweets and ice-cream better than meat and rice. If we eat them    2   (在┉之后) a meal, they are not bad for us. But if we eat them before a meal, they may take away our appetite. It is    3 （重要）for us to eat our meal regularly each day. When we feel    4  （担心的）or    5  （兴奋的）we may not want to eat.         When a man is worrying   6 （关于）something, it is difficult for him to swallow anything dry because he has little appetite. Many years   7（以前）,某DNA片段中胸腺嘧啶的比例为a，则该DNA片段中含鸟嘌呤的碱基对占所有碱基对的比例是                                   （    ）     A．a        B．1—2a        C．2a       D．1／2一a