Why is the verb 選ばれる left in its stem form in the middle of this sentence?


しかし、アメリカではふつう、スポーツチームは腕｛うで｝で選｛えら｝ばれ,学業｛がくぎょう｝のコースは学力｛がくりょく｝によって選｛えら｝ばれます。
However, in America, sports teams are generally chosen by skill, and academic
courses by ability.

