— I'm in _______, _______.
A:grade seven; class three
B:class Three; grade Seven
C:Class Three; Grade Seven
D:Grade three; Class seven
某DNA片段中胸腺嘧啶的比例为a,则该DNA片段中含有鸟嘌呤的碱基对占所有碱基对的比例是
周围高,中间低的地区的地形属于( )