Incandescent bulbs are designed by keeping in mind that the resistance of their filament increases with the increase in temperature. If at room temperature, `100W, 60W and 40W` bulbs have filament resistances `R_(100), R_(60) and R_(40)`, respectively, the relation between these resistances is
A. `1/R_(100)=1/R_(40)+1/R_(60`
B. `R_(100)=R_(40)+R_(60)`
C. `R_(100)gtR_(60)gtR_(40)`
D. `1/R_(100)gt1/R_(60)gt1/R_(40)`