Using the '-se' suffix with vosotros?

Question

In an e-mail entitled: "A new error has been submitted by a user [#252851]", Laura Lawless today (4th May) asked me to repost this here:>Question 6 in the [first] 'StudyPlan' quiz which I did on 3rd May, required a translation for:"You two, wash your hair!"The 'vosotros' part was already provided for us, so I chose:Vosotros dos, ¡A lavaros el pelo! However, I was marked incorrect, because you said that another possible option is:Vosotros dos, ¡A lavarse el pelo!>> But surely the "-se" suffix is not compatible with "vosotros"? … i.e., it has to be '-os', [only '-os', not both].

Accepted Answer

Hola David,
both answers are correct "a lavaros" and "a lavarse". In the lesson we explain that with pronominal verbs we tend to use the infinitive with -se but for a command to "vosotros" you can also use -os at the end. The question in the quiz is a "multichoice" one where it says [one, some or all may be correct]. 
If the student chooses only one correct answer when there are two possible correct answers then you don´t get 100% as you are missing one. 
This is what happened here. Here is the lesson to revise and read about the two possibilities.
I hope this helps.
Saludos
Inma