He can't be ill. He must be tired.#
It can't be my new hat. It must be my old one.#
She can't be Danish. She must be Swedish.#
He can't be a dentist. He must be a doctor.#
She can't be forty. She must be fifty.#
It can't be the 20th. It must be the 21st.#
He can't be the youngest. He must be the oldest.#
He can't be reading. He must be sleeping.&