Two blocks, each of mass m, are stacked on a horizontal floor. The static and kinetic coefficients of friction are μs and μk (with μs>μk) at both the block-block interface and the bottom-block-floor interface. Both blocks are initially at rest. A constant horizontal force F is applied to the bottom block. Which of the following best describes the bottom block's acceleration as a function of F, as F is increased from zero?
AIncreases linearly from the origin with a single constant slope, with no discontinuities or kinks
BZero up to a threshold, then jumps discontinuously upward to a linear segment that continues with a single slope thereafter
CZero up to a threshold, then jumps discontinuously upward to a linear segment, then bends smoothly (no jump) to a steeper linear segment at a second threshold
DZero up to a threshold, then jumps discontinuously upward to a linear segment, then jumps discontinuously upward a second time to a steeper linear segment
EZero up to a threshold, then jumps discontinuously upward to a constant value and remains flat thereafter