The MOSFET transistor is a bidirectional device, but the current can flow througth source to drain, only if the voltage in drian is bigger than source terminal. If you want learn about this devices, you can read microelectronics circuits by sedra and smith.
Added after 33 seconds:
The MOSFET transistor is a bidirectional device, but the current can flow througth source to drain, only if the voltage in source is bigger than drain terminal. If you want learn about this devices, you can read microelectronics circuits by sedra and smith.
Added after 1 minutes:
The true answer is the second, in the first answer a had a little mistake, sorry.