DNA Tm值较高是由于下列哪组核苷酸含量较高所致？
A．G+A
B．C+G
C．A+T
D．C+T
E．A+C
某DNA分子中腺嘌呤的含量为15%，则胞嘧啶的含量应为：
A．15%
B．30%
C．40%
D．35%
E．75%
Sung Ling is in San Francisco to visit______________ .
A. John, Kate'sfriend from school
B. Mr Summers, Kate's neighbour
C. Susan, Grant's wife
D. Kate Miller, her exchange partner
Ling feels nervous because_________________________ .
A. she doesn't want to introduce her to Kate and her parents
B. she wonders she is not good at English
C. she isn't sure how to introduce herself and greet people
D. she likes a limp hand shake with the whole hand
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