I remember when I asked about "everyone" before, Rachel told me it would be better to say "everyone opens their mouth" than "everyone opens their mouths", for everyone has only one mouth. I wonder when it comes to "all" do we still use the singular forms of things like left hand? Thank you.
1. Please raise your left hand if you have any questions.
2. Please raise your left hands if you have any questions.
3. Please all put down your left hand.
4. Please all put down your left hands.