Two Blocks Rest on a Horizontal Frictionless Surface as Shown

Multiple Choice

Two blocks rest on a horizontal frictionless surface as shown. The surface between the top and bottom blocks is roughened so that there is no slipping between the two blocks. A 30-N force is applied to the bottom block as suggested in the figure.
-What is the minimum coefficient of static friction necessary to keep the top block from slipping on the bottom block?

A) 0.05
B) 0.10
C) 0.20
D) 0.30
E) 0.40