(1) When the mercury level rises to m, the gas in B has just been sealed, the volume of the sealed gas is V, and the pressure is the pressure to be measured, while the final state volume of this gas is 14πd2h, and the pressure is p+ρgh, which is obtained by Pomar's law.

pV=(p+ρgh) 14πd2h，

From the meaning of the question: 14πd2h is much less than V, then this formula is approximately: pV=ρgh? 14πd2h，

Can be solved: p =▽GH 2d 24v.

(2) Because when 14πd2h is much less than V, P =π▼GH 2d 24v is obtained, and there is bound to be an error in calculating the pressure P by the approximate method. When V is large or 14πd2h is small, the calculation result is more accurate. Therefore, in order to reduce the experimental error and improve the measurement accuracy, we can appropriately increase the volume V of B or decrease K 1.

So choose a and C.

(3) Because the P measured in this experiment has nothing to do with atmospheric pressure, the change of atmospheric pressure will not affect the measured value of extremely thin gas pressure.

So the answer is:

The expression of the measured pressure (1), p = ρ π gh2d24v.

(2) A and C.

(3) no.