Is there anything which you want?先行词为anything时不是要用that的吗?为什么用which?

这里好像有处不当之处.
根据：
a) 在there be 句型中,只用that,不用which.
b) 在不定代词,如：anything,nothing,the one,all,much,few,any,little等作先行词时,
只用that,不用which.
正确的是：Is there anything that you want?
由于先行词在定语从句当宾语,所以引导词也可以省略：
Is there anything you want?