A thin convex lens made from crown glass (μ=23) has focal length f . When it is measured in two different liquids having refractive indices 34 and 35 , it has the focal lengths f1 and f2 respectively. The correct relation between the focal lengths is:
a. f1=f2<f
b. f1>f and f2 becomes negative
c. f2>f and f1 becomes negative
d. f1 and f2 both become negative