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) gt R_(60) gt R_(40)`
D. `1/R_(100) gt 1/R_(60) gt 1/R_(40)` .
