题干
Dogs are our good friends as they can keep ______ with us when we are lonely．
A：ambitio
B：company
C：balance
D：distinctio
答案(点此获取答案解析）
B
目标函数z＝y－ax(a∈R)，若z取最大值时的唯一最优解是(1,3)，则实数a的取值范围是