This topic studies river systems. In item AD, multinational rivers refer to rivers flowing through the territories of two or more countries. International rivers are multinational rivers that are open to all countries through treaties. The sections of rivers flowing through countries belong to their own territories, and each country has sovereignty over the sections located in its territory. The reach of international rivers flowing through the territory of each country remains the territory under the sovereignty of that country. So A is right and D is wrong.

Item B, each coastal country in a multinational river should take into account the interests of other coastal countries when exercising its rights over the river. Countries shall not make harmful use of rivers, change their flow direction or block their rivers. So b is wrong.

Item C, international rivers generally allow ships of various countries, especially merchant ships, to sail harmlessly. So c is correct.