[填空题]
The lowest pressure attainable using the xgyi - (mn/0ilbest available vacuum techniques is ab6 53z tnyuf+xde 6nee j59mxk(.dthj/out txjz5d6 he/ue9yn jdex.6n3m(5tf k +$10^{-12} N/m^2$ At such a pressure, how many molecules are there per $cm^3$ at 0$^{\circ} $C? $\frac{molecules}{cm^3}$