That a different component part of the vaccine is necessary is principally due to the variable change of ______.

A．concerns the damaging effects of influenza
B．mentions the steps of fighting against the harmful effects of influenza
C．emphasizes the worry expressed by all age groups
D．both A and B

A．all the institutions on the blacklist are arbitrary and negative
B．there is no hard evidence to prove that faster completion rates result from greater efficiency rather than lower standards or less ambitious doctoral topics
C．many of the best students go quickly into jobs where they can apply their research skills, but consequently take longer to finish their theses
D．some polytechnics are penalized by random fluctuations in student performance