By conscience and courage,by deeds of devotion and daring,he soon commended himself to his fellows and his officers; and it was to these qualities and to some knowledge of the country that he owed his selection for his present perilous duty at the extreme outpost.主要是这一句“it was to these qualities and to some knowledge of the country that he owed his selection for his present perilous duty at the extreme outpost."这个是什么用法,it is to.分析一下句子成分!