Diamond is an example of covalent crystal.

Answer

B

Explanation

When liquid just starts forming vapours we have Roult,s law valid with `X_b` and `X_c` as mole fraction in liquid state so equation of curve obtained by collecting such points will be
`P=X_B^lP_B^@+X_C^lP_C^0`
`=X_C^lP_B^0+(1-X_B^l)P_C^0`
`=P_C^0+(P_B^@+P_C^@)X_B^l=P_C^@+(P_B^@-P_C^@)X_B`
The second curve will not be a straight line having equation
`P=(P_B^@-P_C^@)/(X_B^l(P_C^@-P_B^@)+P_B^@)`