Default [Q] No "off" option of Camera Shutter Sound

Dear All

I have a problem. I look at videos of others model of android phone, they have an option to turn off Shutter Sound of Camera (Example: Tone 1, Tone 2, Tone 3, Tone 4, Off) but for my model LG Optimus G E973, there are only 4 options (Tone 1, Tone 2, Tone 3, Tone 4), no "off". Even if I set the phone in silent mode, the shutter still make sound. It's very annoying when I use the phone in my classroom. What should I do?