管道对接焊接时，壁厚超过（）就应开坡口焊接，保证焊道的质量要求。
A、1mm
B、2mm
C、3mm
D、4mm
参考答案：C
There are only ____ new words in the passage, but I know _____ of them. [ ]
A. a few; none
B. a little; none
C. little; all
D. few; all
细菌“核质以外的遗传物质”指( )
A．mRNAB．核蛋白体C．质粒D．异染颗粒E．性菌毛
