Dear teachers,<br/><br/>I tried an online grammar test and I found this which made me visit EF again.<br/><br/>What correction should be made to this sentence?<br/><br/><b>One of their <u>theories</u> is that the first child receives more of the parents' attention than other children so first-borns tend to be more intellectual.</b><br/><br/>And the answer says "change theories to <u><b>theory's"</b></u><br/></br>Could you help me please why "theories" is wrong in this case?<br/><br/>Thanks a lot teachers.