With class A bias, the base current is set so that collector current flows.
Increasing or decreasing base current causes collector current to increase and decrease.

With class B bias, the base current is set to zero. No collector current flows and the transistor is CUT OFF.
The collector current can only be made to increase from this point, not decrease, by increasing the bias.

With class C bias the bias point is set well below cut off.
Collector current can only be made to flow by increasing the bias point considerably above its present value.