I'm teaching English in Japan. I'm using "Grammar In Use Intermediate Workbook" (Cambridge). The following is the question in the workbook:
Complete the sentence. Use can, could, must or (be) able to + verb in parentheses.
- A: You have chest pain? I'll take you to the emergency room. It ( ) (be) something serious.
The workbook says the correct answer is "could".
But students said to me that "must" and "can" also seem to be correct.
I also think that the correct answer will change depending on the context. Could you kindly give me some advice if we can use both "must" and "can" in this situation? What's the difference, and how can I explain it to the students?