The correct answers are C. Academic journals and D. Research papers by experts
Step-by-step explanation:
A research paper is a type of academical document in which the author reports the results and process of research, considering this is a formal, academical paper it is recommended to include only credible sources. The main features for a source to be credible are that the source is objective and updated, written or created only by experts on the topic, supported by evidence and has been checked by other before is published, this means blogs, magazines, personal opinions or experiences or sources written by students are not credible. But academical journals and research papers are usually credible sources because they are written only by experts, they are usually back up with reliable evidence, they are objective and updated and also academical journals and research papers are checked by other experts before being officially published.