问题补充：
This is difficult problem that few students can work it out.
A. so
B. so a
C. such
D. such a
答案：
A. so
B. so a
C. such
(答案→)D. such a
解析：句意：这是如此难的题以至于几乎没有学生算出它。So，如此，那么，用法是so+形容词，such,如此，那么。用法是：such+(a/an)+(形容词)+名词，但名词前有few,little,many,much修饰时只用so，故选D
