重复观测边
参考答案：同一基线边，若观测了多个时段（≥2），则可得到多个基线边长。这种具有多个独立观测结果的基线边，称为重复边。
2x2-9x+8=0．
After two hours the children got to the hill, tired ______ happy. [ ]
A. and
B. but
C. although
D. however
