In the study plan test

In the study plan test

I was presented with the following:

Nous ________ les cheveux.We brushed our hairHINT: Conjugate "se brosser" (to brush) using Le Passé ComposéMy answer was: Nous nous sommes brossés les cheveux.It was marked, Nearly Correct. Kwiziq gave the following as the correct answer: Nous nous sommes brossé les cheveux.I responded to the question by agreeing with number for 1st person plural, (nous + être=past participle + plural)Here is the text from the lesson:Note that when être is used as the auxiliary in compound tenses such as Le Passé Composé, the past participle must always agree in gender and number with the subject of the verb.Please explain why I was not graded as completely correct.Thank you.