问题 good exercise与a good exercise

Swimming is supposed to be _____ very good exercise for older people. 这里到底能不能加不定冠词a？有的人说能加不定冠词，有的人说不能加不定冠词。请教各位。谢谢！

话题 every once in a while

话题 be in for a shock

话题 a full-fledged home

话题 A King in New York

话题 a little bit late home

话题 a home away from home

话题 One student in a hundred

话题 while still a young man

话题 within a couple of years

话题 the homecoming of a lifetime

话题 a great deal of me.

话题 there came a turning point.

话题 have a good shot at

话题 with a knife and fork