Extract indices after comparing two vectors

A <- c(1,2,3,4)
B  <- c(0,3,1,5)
which(A<B)
max(which(A<B))

Dear All,
I would like to ask a question about how to locate the indices after comparing two numeric vectors.
Say, I have,  A <- c(1,2,3,4) and  A <- c(0,3,1,5)
and we cmpare:

 idx <- A<B

So:

idx

[1] FALSE  TRUE FALSE  TRUE

Question 1:
I would like to get a vector that stores the indices for entries of A such that this relationship is true. In this case,
a vector  idx1 < c(2,4), because the 2nd and 4th entries of A satisfy the conditions

Question 2:
I would like to know the maximum index of the entry in A for which the relationship is true. In this case,
max_indx = 4, since the 4th entry of A is such.

Could you please give me some hints?

Thank you,
-Chee

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