Son fuentes con una estructura y sistemas de búsqueda muy efectivos que contienen material académico producidos por los miembros de una institución (o varias).

Explanation:

They are sources with a structure and very effective search systems that contain academic material produced by the members of an institution (or several).

wheres the

changeable lenses of different focal lengths allow light to enter the camera, which then forms the image on light-sensitive film" is the one among the following statements about working principle behind a basic still camera is true.