There is an acro series with a minimum of two flight elements requirement. Any A B or C element with flight is allowed or one D/E which will receive C credit.
However, if you choose to compete the minimum requirement, say a B+C acro series, you will likely incur an up to .2 deduction for not being up to the competitive level. Unless, of course, the majority of level 9s in your area are all competing the minimum. But if other girls are competing a C+C or a B+B+C or a B+C+C, you will get the deduction.
There are also compositional deductions for not going two different directions which is why you often see a single acro skill, but a single acro skill is not a requirement.
Hope this helps.