A they are useless
B in paper chase
C that it is easily reached
D that different people have different requirements
E they are not comfortable
F in investing in coupons
You can put your file cart anywhere you like, on condition______

A．Right after cell phones were invented.
B．Before the number of cell phone users reached a critical mass
C．When cell phone users totaled to a certain number.
D．When the number of cell phones decreased to a certain number.