In this task, find the most appropriate number to replace the blank (indicated with _ ) and express it in words.

Q: Echocardiography demonstrates in clear, graphic fashion the _ cardiac chambers and four valves.

A: four
****
Q: Most smokers try to quit at least _ or six times before they succeed, researchers say.

A: five
****
Q: Radiologists train three to _ years to improve their perception.

A:
four
****