—Is Linda a nice girl in your class?

—Yes, she is. She always helps us with math.[ ]

A. helps us learn math  

B. helps us learning math  

C. helps math with us  

D. helps learn math

下列变化属于物理变化的是（　　）

A．

鱼池喷水溶解氧气

B．

遇火碱的鸡爪萎缩

C．

溶洞中石笋的增长

D．

纸遇浓硫酸变黑