As is fine, Now suggests that there were able to but now aren't, and Given isn't normally used with the 2nd person. I can't think of a reason, it just seems rather odd to me (I'm British). In the last sentence 'Now that' doesn't work, the others do.
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Anonymous2.Now that you are unable to answer, perhaps we should ask someone else.In view of my post above, let's go over this example once again.