Which architectural element, sometimes seen in early christian longitudinal churches, did not derive from the roman basilica? (one word)?

Achair of height 1 m is placed 1 m in front of a plane mirror. a girl is standing 3 m behind the chairas shown. find the minimum length of the plane mirror in order that she can see the whole imageof the chair.a. 0.50 mb. 0.75 mc. 0.80 md. 1.00 m