Put in 'must' or 'can't': Where's Lucy? She _______ be at the library, as she often goes there at this time.

Where's Lucy? She ____must___ be at the library, as she often goes there at this time.

Dịch: Lucy đâu? Cô ấy phải ở thư viện, vì cô ấy thường đến đó vào giờ này.