It is fairly common in Africa for there______ a group of expert musicians surrounded by others who also join in the performances.

A．being

B．to be

C．be

D．is

关于K-B纸片扩散法结果判读的描述，下列说法错误的是

A．平板置于黑色无反光背景
B．抑菌圈的边缘以肉眼见不到细菌明显生长为限
C．参照标准解释结果
D．培养基中如果加有血液需打开皿盖
E．加有血液的培养基应借透射光从正面测量抑菌圈