for people on a particular kind of Medicare plan for prescription drugs (= ones that can only be bought if a doctor orders them for you), the fact of having to pay for a drug because its cost is between two levels at which the cost will be paid by their plan

Many seniors fall into the donut hole with Medicare Part D. 许多老年人在医疗保险D部分陷入“甜甜圈漏洞”。

To avoid that gap, known as the donut hole, people should look at the brand-name drugs they take and see if there are generic alternatives. 为了避免这种被称为“甜甜圈洞”的漏洞，人们应该注意自己服用的品牌药物，看看是否有非专利替代品。

a small ball-shaped cake, originally cut out from the middle of a donut (= a small circular cake with a hole cut out of the middle, fried in hot oil)

I just ate about 12 chocolate donut holes. 我刚吃了12个巧克力甜甜圈球。

Don't leave without trying the fried-to-order donut holes. 离开前，一定要尝尝这些炸甜甜圈球。