Single/multi-select question: Does the sentence "This may be modified by adding low-density materials such as boron, which lowers the index of refraction (see crown glass), or increased (to as much as 1.8) with high-density materials such as (classically) lead oxide (see flint glass and lead glass), or in modern uses, less toxic oxides of zirconium, titanium, or barium." provide a valid answer to the question "What can be used to make glass less refractive?"
OPT:
(a). yes.
(b). no.
(a).