沙生植物对流沙环境活动性的适应途径主要是（）和（）。
参考答案：避免风蚀;适度沙埋
Thousands of athletes______ in many countries will compete _______ medals in the 2008 Olympic Games in Beijing.
A．involved; in
B．involving; by
C．to involve; against
D．involved; for
美苏争霸期间，两国力量此消彼长、相互制衡。下列表明苏联退缩的史实是[ ]
A．结束越南战争
B．执行“莫洛托夫计戴”
C．撤出古巴导弹
D．签订《华沙条约》
