Convex Lens Colour

A convex lens is placed on a flat glass plate and illuminated from above with monochromatic red light. When viewed from above, concentric bands of red and dark are observed. What does one observe at the exact center of the lens where the lens and the glass plate are in direct contact?

a. a bright spot that is some color other than red
b. a dark spot
c. a bright red spot
d. a rainbow of color