C. if either I or II is implicit.
Feedback The author of the statement is astonished. A person is astonished only when he sees a surprising thing. The author was surprised by the incident. But we do not know whether he believed or disbelieved it. If the latter is true, (I) is an assumption, and if the former is true (II) is an assumption. But both cannot be assumptions simultaneously. Hence, either (I) or (II) is an assumption that follows.