相互了解，是发展国与国之间关系的前提。唯有相互了解，才能增进信任，加强合作。
A. Mutual understanding is the basis for state-to-state relations. Only with mutual understanding, it would be possible for countries to build trust and promote cooperation with each other.
B. Mutual understanding is the basis for state-to-state relations. Without it, it would be impossible for countries to build trust and promote cooperation with one another.
C. Mutual understanding is the basis for state-to-state relations, without it, it would be impossible for countries to build trust in and promote cooperation with each other.
D. Mutual understanding is the basis for state-to-state relations, only with mutual understanding, it would be possible for countries to build trust in and promote cooperation with one another.