2g氦气的物质的量为2g 4g/mol =0.5mol, A.2g氢气的物质的量为2g 2g/mol =1mol; B.3.01×1023个CO分子的物质的量为3.01×1023 6.02×1023mol?1 =0.5mol; C.0.25molN2; D.标况下状况下5.6 L CH4的物质的量为5.6L 22.4L/mol =0.25mol,根据V=nVm可知,同温同压下,气体与2g氦气所占体积相同,则气体与氦气的物质的量相等,故选:B.