Does anyone know what a, b,c or d is

b mirrors submit a iisner? e. for which optical devices does d sometimes have a positive value? o a a and b o b. a and c o c. b and d 0 d. b and cb

conduction, convection, radiation

force required to detach it is given by

charges are taken to be fundamental charge

now we have

so above is the minimum distance at which it will detach