To determine which sentence should be left out of an abstract, look for information that is either too detailed, tangential, or not essential to understanding the main findings and significance of the article. Typically, introductory phrases, background information, detailed methodology, or specific examples can often be omitted in an abstract.
If you provide the specific sentences or context from the article, I'd be glad to help you identify which one may be extraneous or less critical for an abstract!