For a second Rudolf thought his host [the lighthouse keeper] might be joking； but the serenity o f the other's expression reassured him.
A....but Rudolf found from the old man's expression that he was not serious. B....but Rudolf's expression showed the old man that he was only joking. C....but the old man s calm expression convinced Rudolf that he was not joking. D....but Rudolf looked peaceful, so the old man was sure that he meant what he said.正确答案C
