肉毒毒素的致病机制是
A．抑制细胞蛋白质合成B．阻碍乙酰胆碱释放C．激活腺苷酸环化酶D．使自主神经麻痹E．使脑神经和外周神经兴奋性增强
参考答案：B
A group of young Americans have come to Beijing for a visit. Half of them, ___is, 50 persons, can speak Chinese fluently.
A．that
B．which
C．who
D．it
He failed to_________ himself to the new environment. [ ]
A. adapt
B. attach
C. adopt
D. advocate
