the sun rises in the east and sets in the west ,so it seems as if the sun____ round the earth

as if后一般用虚拟语气：1,.与现在事实相反用过去式（be动词用were）,本质上讲就是原来的时态退后一格,此处就是这情况,本来应the sun circles round the earth.（退一格即circled）或The sun is circling round the earth,用进行时态更有动态的感觉,（退一格即were circling,注意be动词用were),故此题选A,但就语法而言B也是正确的,只是A更形象生动.
2.与过去相反 had done （本质上讲也是原来的时态退后一格）3.与将来相反would/could/..do
另外若是很可能出现的情况或是事实,也可用正常时态,如It seems as if it's going to rain.