18.(导选题,5,0分)-|||-在下列无穷级数中,收敛的级数是 ()-|||-A sum _(n=1)^infty (sqrt (n+1)-sqrt (n));-|||-B sum _(n=1)^infty ((dfrac {n)(n+1))}^n-|||-sum _(n=1)^infty ln (1+dfrac (1)(n));-|||-D sum _(n=1)^infty dfrac ({(-1))^n-1}({n)^2+1}.