Magma is classified asasaltic, andesitic, or rhyolitic based on what factor

The three classifications of magma are mainly identified using their silica composition. Basaltic magma has the lowest silica content of 45-55% by weight. This makes it the least viscous and least explosive. Andesitic magma has 55-65% , making it more explosive and viscous. Rhyolitic magma has 65-75% silica, and is the most viscous and catastrophic

It is B balance, because it has a 0,01 g divisions scale. The minimun division in A balance is only 0,1 g, i.e, decigram measurement.