Elena is a cybersecurity engineer. Recently, several employees received emails that appeared to be from the company's HR department asking them to click a link and enter personal details as well as confidential information about the company. Questioning the employees, Elena discovered that the emails had spelling errors and the sending email address was not formatted as is typical for the company. Although the emails were from a malicious source, the real problem seemed to be a lack of employee training. Given human weakness as a driver for potential security risks, what has Elena determined is the primary problem? a. Hackers b. Insiders c. Human mistakes d. Social engineering