sentence2_structure: Reversible reactions<>are indicated<>by opposite-headed kinds of arrows
sentence1: Reversible reactions are symbolized in chemical equations by a double-headed arrow, but the standard remains to call the materials on the left the reactants and the materials on the right the products.
sentence2: Reversible reactions are indicated by opposite-headed kinds of arrows.
question: Reversible reactions are indicated by what kinds of arrows?