Something is simple if it doesn't have a lot of elaborate parts, or is very easy to understand. The picture is of a very simple flower arrangement.
The opposite of simple might be elaborate, complicated or difficult.
Single is used to describe just one of something or someone. For example, if you're not in a relationship, you can say you're single.
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You sip a drink when you drink only a very small amount at a time. When you sip your drink, you tend to drink it very slowly.