Her parents _____ at home.[ ]
A. is
B. isn't
C. must
D. aren't
答案:D
细菌抵抗不良环境的特殊存活形式是()
A.芽孢
B.荚膜
C.菌毛
D.鞭毛
E.细胞膜
饱和热水塔若液位()将增大系统压降,有时还会造成半水煤气带水,引起触媒温度骤变,多次带水将使触媒粉化和系统阻力增大。
A.过高;
B.过低;
C.波动;