Read the passage and choose the best answer

I think it is necessary for students to wear uniforms when they are at school.
Firstly, uniforms encourage students to be proud (14)_________ being students of the school they are going to because they are wearing the uniforms with (15)_________ bearing their school’s name.
Secondly, wearing uniforms helps students (16)_________ equal in many ways. Everyone starts (17)_________ the same place, whether they are rich (18)_________ poor. They are really friends (19)_________ one another under one school roof.
Finally, wearing uniforms (20)_________ practical. You don’t have to think of what to wear every day.
Therefore, all students, from primary to high (21)_________ should wear uniforms.

Câu 14 : Firstly, uniforms encourage students to be proud (14)_________ being students of the school

A. of

B. in

C. at

D. on