請回答第 15 題至第 18 題：
         Subjects in many old photos were not smiling at the camera for two reasons. Ann Thomas, curator of photographs atthe National Gallery of Canada, says, “In the early days of photography,   15   existed a tradition of formality－handeddown from portrait painting－in which posing for a photograph was considered a   16   occasion.”
         The second reason, according to Thomas, is that prior to the 1880s, photographs required long exposures. So   17   theprocess, this could involve 20 minutes or more. This necessitated the subject remaining still, and since maintaining asmile for such an   18   period was difficult, it was discouraged. The arrival of smiling faces in photos came after theinvention of the snapshot in the 1880s with the introduction of the Kodak dry-plate camera.
【題組】15
(A) it
(B) that
(C) when
(D) there