Complete the exercise by providing the plural forms of the nouns.

{"exercise": [ {"number": "1", "singular": "one balloon", "plural": "four balloons"}, {"number": "2", "singular": "one tree", "plural": "two trees"}, {"number": "3", "singular": "one cat", "plural": "three cats"}, {"number": "4", "singular": "one frog", "plural": "two frogs"}, {"number": "5", "singular": "one bird", "plural": "four birds"}, {"number": "6", "singular": "one star", "plural": "three stars"}, {"number": "7", "singular": "one book", "plural": "two books"}, {"number": "8", "singular": "one cow", "plural": "five cows"}, {"number": "9", "singular": "one chair", "plural": "four chairs"}, {"number": "10", "singular": "one rabbit", "plural": "three rabbits"}, {"number": "11", "singular": "one doll", "plural": "two dolls"}, {"number": "12", "singular": "one ball", "plural": "five balls"}, {"number": "13", "singular": "one mouse", "plural": "three mice"}, {"number": "14", "singular": "one baby", "plural": "two babies"}, {"number": "15", "singular": "one box", "plural": "four boxes"}, {"number": "16", "singular": "one leaf", "plural": "six leaves"}, {"number": "17", "singular": "one brush", "plural": "four brushes"}, {"number": "18", "singular": "one knife", "plural": "two knives"}, {"number": "19", "singular": "one glass", "plural": "three glasses"}, {"number": "20", "singular": "one bus", "plural": "four buses"} ]}