(2013·安徽省芜湖三校一模)A recent study has found that the number of smokers ________ sharply over the past five years.

A．is increasing

B．are increasing

C．has been increasing

D．have been increasing

合并同类项

（1）3a-5a+6a．             （2）x²y+4x²y-6x²y．

（3）-3mn²+8m²n-7mn²+m²n．   （4）2x³-6x-6x³-2+9x+8．