Why is the spanish civil war called a "proxy war"?

 a 'proxy war' means a war which is instigated (or in other words 'brought about') by major powers that do not become involved. Therefore, the big powers such as USSR and Nazi Germany aided both sides in Spain, however, weren't involved (fighting battles with their own army). Im assuming the answers are the same ones I had last year. So the Answer is D if It's multiple choice.