I’m sure it wasn’t Mr Phong you saw because he’s in Ha Noi.
A. It couldn’t be Mr Phong you saw because he’s in Ha Noi.
B. You mustn’t have seen Mr Phong because he’s in Ha Noi.
C. It can’t have been Mr Phong because he’s in Ha Noi.
D. Mr Phong is in Ha Noi, so you might have seen him.