题目

Their cheery song brightens many a winter's day. But robins are in danger of wearing themselves out by singing too much. Robins are singing all night一as well as during the day, British-based researchers say.     David Dominoni, of Glasgow University, said that light from street lamps, takeaway signs and homes is affecting the birds' biological clock, leading to them being wide awake when they should be asleep.     Dr Dominoni, who is putting cameras inside nesting boxes to track sleeping patterns, said lack of sleep could put the birds’ health at risk. His study shows that whe已知函数f（x）是定义在区间[-a，a]（a＞0）上的奇函数，且存在最大值与最小值．若g（x）=f（x）+2，则g（x）的最大值与最小值之和为（　　）A．0B．2C．4D．不能确定