Which phrase accurately characterizes sexually transmitted infections (STIs)? are caused by bacteria, viruses, or parasites
always cause visible symptoms
are rarely transmitted by sexual contact
can be contracted only by women

Are caused by bacteria, viruses, or parasites, because not all STI’s will show visible symptoms, which is why you should be very careful. They are not rarely transmitted by sexual contact, but they can be transmitted other ways. And males can also have an STI.

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The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)