At, In, On; father’s friends, father’s friend, father’s______ th

At, In, On; father’s friends, father’s friend, father’s
______ the firstday of my visit, we went out with one of my ______for lunch at an outdoor café.
【正确答案】：On / father’s friends
【题目解析】：The genitive is generally formed in writing by adding ’s to singular nouns and to the plural nouns that do not end in -s. Plural nouns that end in -s take apostrophe as genitive marker.