\text { Let } u=x^{2}+3 . \text { Which equation is equivalent to }\left(x^{2}+3\right)^{2}+21=10 x^{2}+30 \text { in tems of } u \text { ? } \text { A) }

u^{2}+10 u+51=0 \text { B) } u^{2}-10 u+51=0 \text { c) } u^{2}+10 u+21=0 \text { D) } u^{2}-10 u+21=0