I don't understand the 1st exercise : <span style="color: rgb(128, 128, 128); font-family: arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; ">Have a go at this exercise:</span><div class="box exercise-box" style="margin: 0px 0px 20px; padding: 20px; background-color: rgb(231, 231, 231); color: rgb(51, 51, 51); font-family: arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; ">Identify the mistake in the following sentences and correct it. There is only ONE mistake, so you should not change more than ONE of the options: (...)<div class="box exercise-box" style="margin: 0px 0px 20px; padding: 20px; background-color: rgb(231, 231, 231); color: rgb(51, 51, 51); font-family: arial, helvetica, sans-serif; font-size: 14px; line-height: 20px; ">All my responses was OK but the machine told me " not done" ....