按照原油含油量不同，可以把原油分为三类，即低流原油，含硫原油和（）原油.
参考答案：含硫，高硫原油
The reason _____ he explained is ___ she was ill.
A．that , that
B．which , that
C．why,because
D．why, that
个案社会工作有哪些主要特征？
