Select the correct text in the passage.

Which phrase from the text excerpt explains the Duke's motivation for allowing Valentine to stay?

The Duke of Milan, as was the custom in those days, thought himself at perfect liberty to dispose of his daughter in marriage as best pleased himself, with but scant regard for her own feelings on the subject. He suspected there was some love between Silvia and Valentine, and saw many fittle things when they thought him blind. He often determined to forbid Valentine his Court and his daughter's company, but, fearing that his jealousy might perhaps be leading him into error, and that he might bring disgrace unworthily upon Valentine, he resolved not to act rashly, but by gentle means to try to discover the truth. In the meanwhile he kept a strict watch over Silvia, and, fearing some attempt on the part of the young lovers to escape secretly, he gave directions that Silvia should be lodged in an upper tower, the key of which was brought every night to himself.