关于虚拟语气的 .All matter,whether it ---- gas,liquid or solid,is m

关于虚拟语气的 .
All matter,whether it ____ gas,liquid or solid,is made up of atoms.
A be B was C will be D were
想问一下为什么这里是选动词原形BE?提示一下是什么语法谢谢