person that thier arms

[zaed_salah]

hi all Teacher

I met this sentce in 'Cambridge Dictionary Online':
Quadriplegic; means 'a person who is permanently unable to move any of their arms or legs, often because their spine has been injured'.
Here, why they uses plural in Thier arms and thir spine??
is it correctif we say' person who is permanently unable to move any of his/ here arms or legs, often because his/ here spine has been injured'.

I am not sure but I read somewhere that we often use plural when we talk about persons in general, and that is formal, also to avoid the " he or she " !!. right??

Thank you

[Tdol]

It's an area where different people do thhings differently"
A person cannot move his limbs (traditional way)
A person cannot move his or her limbs
A person cannot move their limbs

I use 'their' because its quicker and simpler when referring to a singular person of either gender. There are those who argue that it is numerically inaccurate, but I think this is an oversimplification of singular/plural in English ('everybody' has a plural meaning and tkaes a singular, etc).