If memory cells are always present after the first exposure to a virus, then why do people get the flu every year?

Which of the following is an oxidation-reduction reaction? (pictured below)

Why is it important to reduce the number of chromosomes in half when forming gametes for sexual reproduction?

Tara sees a commercial for a brand of pills that promise to boost energy and people perform better in work and school. as people in the commercial talk about how much better they feel when taking the pill, tara notices a message on the bottom of the screen that reads, "this pill is not approved by doctors and has not been tested in controlled experiments." what is the best conclusion that tara should make about the commercial? it is making scientific claims based on subjective observations. it is making scientific claims based on opinions instead of pseudoscientific facts. it is making pseudoscientific claims based on objective observations. it is making pseudoscientific claims based on opinions instead of scientific facts.