Please reorganize the order of the following conclusion and fill in the blanks.1. It can effectively identify the critical components of road network. According to graph theory, two kinds of structural design optimization for road network are proposed to improve the road network robustness. 2. The network efficiency model is used to define the structural vulnerability of road network in this paper. 3. How to combine this model with the dynamic flow in road network is an interesting future research direction.4. However, this study only considers the structure of road network attributes (connectivity), regardless of the dynamic flow in road network. 5. The framework of the road network structural vulnerability measurement and improvement model is proposed. The actual network vulnerability analysis is carried out on Shanghai freeway network.16. Thesis or summary of arguments ______17. Major findings______18. Implication for findings ______19. limitations______20. Further research ______请将答案分别填入16-20题中。（仅填数字即可）现在请填写第16题答案。