Question 17 of 20 Match the exercise with the muscle or muscle group it exercises. Concentration Curl ? Pectorals Pushdowns 2 Quadriceps Bench Press ? Biceps Leg Extensions ? Triceps​

Ithink the answer is c.  hope this is right and

What happens to the influenza virus every 20-40 years and why?

Immunization is the introduction of a foreign substance to provoke  the immune response system.  immunizations are the same as vaccinations. it introduces foreign (but dead/ weakened) illnesses to train the body to fight against them. specific viruses and bacteria have specific antigens, antigens act sort of like name tags. these name tags the immune system recognize what's dangerous to the body and what's safe.