At `t=0`, an inductor of zero resistance is joined to a cell of emf `varepsilon` through a resistance. The current increases with a time constant `tau`. The emf across the coil after time `t` is
A. `varepsilont//tau`
B. `varepsilon(1-e^(-t/tau))`
C. `varepsilone^(-t//tau)`
D. `varepsilone^(-2//tau)`
