There are 15 incomplete sentences in this section. For each sentence there are four choice

There are 15 incomplete sentences in this section. For each sentence there are four choices marked A, B, C and D. Choose one answer that best completes the sentence and blacken the corresponding letter on the Answer sheet.

6. ----Bradford graduated from college with honors at a very young age.

----He _____ have been an outstanding student.

A must B could C should D might

请帮忙给出正确答案和分析，谢谢！