Research refers to the process of looking for information or study something to solve a question, a problem or establish facts that help the researcher understand some issue better or improve some process. On the other hand, sampling refers to the process of using a part of the information gathered during the process of research and use this information to establish general conclusions, this implies sampling is about using one section of the information and taking it to a wider population that determine the research results. Due to this, if the research selects the sampling or the information that will determine the results conveniently, which means he does not select them randomly but on purpose, this will probably lead to a biased results, which means the results are not objective but were influenced by the researcher opinions and therefore are not reliable. Thus, it is true that is a convenience sampling leads to biased research results.
it is the same as the topic, because central idea and topic are both basically the same thing as main idea, which is what the text is mainly about. hope this helped
