So this should immediately look like a problem because we know, in fact, that methane is tetravalent, and this is telling us it's only divalent.
显然这看起来是一个问题，因为我们知道实际上，甲烷是四价的，而这告诉我们它仅仅是二价的。
2
A tetravalent meningococcal meningitis vaccine (licensed in the United States) and a heptavalent vaccine protecting against meningitis A and C (to be licensed in 2007);
Conclusion the developed synthetic medium and purification procedure were feasible and provided an experimental basis for the preparation of tetravalent meningococcus polysaccharide vaccine.