如果将四面颜色不同的小旗子挂在一根绳子上，组成一个信号，那么这四面小旗子可组成______种不同的信号．
4×3×2×1=24，
故答案为：24．
血液保存液主要成分不包括（）
A.枸橼酸三钠
B.枸橼酸
C.葡萄糖
D.氯化钠
E.磷酸盐
Which of the following would be a likely theme for the next paragraph
A．Naturally changing ecological conditions.
B．Animals that have become extinct.
C．Achievements of the government Endangered Species Program.
D．Programs that have destroyed natural habitats.
