培育无子西瓜所依据的遗传学原理是[ ]
A．基因突变
B．基因重组
C．染色体结构变异
D．染色体数目变异
答案：D
He gained his by printing of famous writers.
A．wealth; work
B．wealths; works
C．wealths; work
D．wealth; works
下列哪项是肺通气的动力（）
A.肺泡表面活性物质的多少
B.肋间内肌的舒缩活动
C.呼吸肌的舒缩引起肺内压变化所致
D.支气管平滑肌的舒缩
E.肺内压和胸膜腔内压之差
