Final answer:
There are more than 20 known STIs today, with genital herpes and trichomoniasis often going unnoticed due to being asymptomatic. While condoms reduce the risk, they cannot completely prevent all STIs.
Step-by-step explanation:
The question regarding the number of known sexually transmitted infections (STIs) today can be answered with option D, which states there are more than 20 known STIs currently identified. We understand this based on various sources, such as the Centers for Disease Control and Prevention, which track and report on the prevalence and types of STIs.
Additionally, it is worth noting that STIs can be asymptomatic, meaning that individuals may not show symptoms. Two STIs that often go unnoticed due to the lack of symptoms include genital herpes and trichomoniasis. Moreover, while condoms can significantly reduce the transmission risk of STIs, it's crucial to state that their proper use cannot completely prevent the transmission of all STIs, which makes education and awareness even more important.