[填空题]
The critical density nm r8qruhq(kong+wpzj3t *f/. a2 m 22for closure 6pk.+h2j+ddwnzi 4ki0w nn l:bn4b.e of the universe is $\rho_c \approx 10^{-26}\ \text{kg/m}^3$. State $\rho_c$ in terms of the average number of nucleons per cubic meter. $\ \text{nucleons/m}^3$
