Read this: Many types of sports equipment are made of wood, or were constructed of wood in the past. For example, cricket bats are typically made of white willow. The baseball bats which are legal for use in Major League Baseball are frequently made of ash wood or hickory, and in recent years have been constructed from maple even though that wood is somewhat more fragile. NBA courts have been traditionally made out of parquetry.

Ash and hickory are often used for baseball bats to comply with the rules of what organization?
What is the answer? (If it cannot be answered, return "unanswerable")
Major League Baseball