The old man was found_____a cold morning. A. in B. at C. on D. /选哪个,为什么?in、on、at的用法是什么啊

Question

The old man was found_____a cold morning. A. in B. at C. on D. /
选哪个,为什么?in、on、at的用法是什么啊

Answer 1

C.on 再早晨 in the morning 但如果morning前有定语修饰时（此处是cold),把in改成on