Explanation/Reference:
Explanation for A:
Impact of Selective Authentication
Because all verification of incoming interforest authentication requests is done locally on the receiving domain controller in the trusting forest, access to resources in the trusting forest is likely to be extremely limited for a broad set of users on the network (which is the purpose of this security setting). Consequently, implementing selective authentication might require user education, particularly due to the following reasons:
Users browsing network resources through My Network Places to resources located in a trusting forest

might get access denied messages when attempting to access those resources.
Resources in the trusting forest that were once available to users in a trusted forest might no longer be

available.
References:
http://technet.microsoft.com/nl-nl/library/cc755321%28v=ws.10%29.aspx