8 单选(10分) 极限 (x,(y)_(n)^n-1)cdot dfrac (5cos x)(sqrt {{x)^2+(y)^2+(z)^2}}= () ,-|||-○ A.3-|||-○ B. https:/img.cdnjtzy.com/zyb_2769a757524efd3529c6ba1239aff0a4.jpgsqrt (5)-|||-○ C. ∞-|||-○ D. sqrt (5)