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when doing small signal (AC) analysis.. only alternating (that change with time) voltages are considered. since DC voltage source is not varying (fixed value) it is considered zero and is short circuited to ground.
similarly if its a DC current source it is open circuited..
While doing small signal analysis (AC) we are bothered about the small changes in the circuit(remember there is no change in your DC supply which is large compared to your i/p and hence called large signal analysis.) so we make the DC sources effect on the circuit zero by shorting voltage sources and open circuiting current sources.....