Some cows live on a grassland. $10$ cows can eat all grass in $9$ days. $12$ cows can eat all grass in $7$ days. The amount of new grass that grows each day is constant. $24$ cows can eat all grass in~\uline{~~~~~~~~~~}~days. 
选项为:A:$$2$$  B:$$3$$  C:$$4$$  D:$$4.5$$  E:$$5$$  请回答其中一个选项.