(2) Set D represents a set with the common point of straight line 2x-y= 1 and straight line x+4y=5 as elements;

∴d ∴d={( 1, 1)} is the proper subset of C.

③A={3，a}，B={ 1，4}

When a= 1, A∪B={ 1, 3,4}, a ∪ b = {1}; When a=4, A∪B={ 1, 3,4}, a ∩ b = {4};

When a≠ 1 and a≠4, A∪B={ 1, 3,4, a}, A∪B=? .

④