细菌内毒素的特征是
A．只有革兰阴性细菌产生B．少数革兰阳性细菌产生C．细菌在生活状态下释放D．抗原性强E．不耐热
参考答案：A
So many directors ________, the board meeting had to be put off.
A．were absent
B．been absent
C．had been absent
D．being absent
圆盘造球机与圆筒造球机相比其缺点是（）。
A、生产率低
B、成球不均匀
C、操作不稳定
