[填空题]
The lowest pressure attainable using the z1ctel m(vk5+ best available vacuumon3t j-gcn 0b9 techniques is abojtb90c-ng o 3nut $10^{-12} N/m^2$ At such a pressure, how many molecules are there per $cm^3$ at 0$^{\circ} $C? $\frac{molecules}{cm^3}$