问题如下：
The decrease in trade surplus is most likely caused by:
选项：
A.A decrease of private savings over physical capital invesment.
B.A decrease of government spending over tax revenue.
C.an increase of net export.
解释：
A is correct. The trade balance equation is X-M=(S-I)+(T-G)
Trade surplus is when X-M>0, which will decrease when private savings over physical capital investment (X-M) is decreasing or tax revenue over goverment spending (T-G) is decreasing.
考点：贸易平衡方程式
解析：
贸易平衡方程为X-M=(S-I)+(T-G)
贸易顺差表示为X-M> 0，私人储蓄相对于实物资本投资(S-I)减少或税收相对于政府支出(T-G)减少都会贸易顺差会减少。因此只有A选项正确。
net export是指m－x吗
