If \( a_{1}, a_{2}, a_{3} \ldots \ldots . \). are in A.P. and \( b_{k}=a_{k}+a_{k+1}+\ldots \ldo...
If \( a_{1}, a_{2}, a_{3} \ldots \ldots . \). are in A.P. and \( b_{k}=a_{k}+a_{k+1}+\ldots \ldots+a_{k+n-1} \) \( (k=1,2,3, \ldots) \), then \( b_{1}+b_{2}+\ldots+b_{n} \) is equal to
(a) \( n(n+1) a_{n} \)
(b) \( (n-1) n a_{n} \)
(c) \( n^{2} a_{n} \)
(d) \( (n+1)^{2} a_{n} \)
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