请解这题weallknowthat_____orangejuiceis____healthydrink.weallknowthat_____orangejuiceis____healthydrink.：A/;a请帮我解释一下第一空为什么不用冠词,第二空为什么a

　　请解这题weallknowthat_____orangejuiceis____healthydrink.

　　weallknowthat_____orangejuiceis____healthydrink.

　　：A/;a

　　请帮我解释一下第一空为什么不用冠词,第二空为什么a