SEOUL, South Korea (AP) — The leaders of North Korea and China have marked the 75th anniversary of their diplomatic relations by exchanging messages that expressed hopes for stronger ties, as outsiders raised questions about their relationship. Sunday’s message exchange between North Korea’s Kim Jong Un and China’s Xi Jinping came as North Korea and Russia have been sharply expanding their cooperation while China apparently keeps its distance. Experts say that the level of exchanges and commemorative programs between North Korea and China in coming months will provide a clue to the exact status of their ties.