题目

Thousands of hungry birds left the countryside of Britain in the bitter winter to find food and warmth in urban gardens. The move included species such as the redwing and the fieldfare, almost unknown outside rural areas. The hard situation of farmland birds has been revealed in the latest survey of bird populations from the Royal Society for the Protection of Birds released today. The redwing has an obvious stripe over the eye and a red flash on the flanks (侧翼), while the fieldfare has a grey head, a peach under­belly, and a distinctive dark band on the tail. Graham Madge, the soc化学反应aM+bN====cW+dE，已知20 g M和10.5 g N恰好完全反应，生成16.5 g E，则M与W的摩尔质量比为(    )