Chọn phương trình đúng khi nói về khí nito và khí hidro A. N2 + 3H2 −to→ NH3 B. N2 + H2 −to→ NH3
C. N2 + 3H2 −to→ 2NH3 D. N2 + H2 −to→ 2NH3

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).

Ineed a picture or something 

The answer is  a: x-rays—2 and ultraviolet rays—3.  hope i !