>Thank you for the replies of you and Rick. I failed in viewing available resources on C both from Explorer on A (or using NET VIEW \\IpAddress). In all locations I logged in using the same account that exists in all servers using the same password. No trusting relations were set up among servers.
>
>The things strange are
>1. There should not be problem server C because it can be connected from office B.
>2. There should not be problem with the computer of office A because it can connect to server B.
>
>If both server C and the computer of office A are normal, why can't the computer of office A connect to server C?
>

There are a number of reasons why this could be the case. The most obvious is if multiple network cards are used to segment the network. Going from A to B, you could be going one way through the network, traversing a different set of network nodes. Each of those nodes is confgiured with the correct protocols. Going from B to C, the same could be true. However, going from A to C could introduce an entirely different route.

Have you tried the other way: going from C to A. If it is an authentication issue, this direction may work. If it is a protocol/routing issue, then this should fail as well.