What is the difference between SEM and SEO?

A SEM is the long-term strategy; SEO provides immediate results through paid placements B SEM focuses on social media; SEO focuses on search engines C SEO is the practice of optimizing websites to rank organically in search results without paying for placement; SEM is a broader term encompassing paid search advertising (PPC) and sometimes includes SEO D They are identical terms; SEM is simply the British English version of SEO