A block of mass m is sandwiched between a rough vertical wall and a horizontally oriented spring with spring constant k. If the spring is compressed a distance x beyond its uncompressed length, then the minimum coefficient of static friction for which the block does not fall is
a) kxmg; b) mgkx; c) mg+kxkx; d) mg+kxmg.
[Figure: Shows block between vertical wall and horizontal spring]