【单选题】

It is a popular notion that autumn leaves are tinted by freezing temperatures. In truth, the foliage is dulled, not colored, by frost. Red leaves such as maples are brightest when sunny days are followed by cool--but not freezing--nights. Under such conditions, sun- made sugars are trapped in the leaves, where they form the red pigment anthocyanin. Leaves that appear yellow in autumn are no less yellow in spring and summer. However, in spring and summer the yellow pigments--carotenoid and xanthophyll--are masked by the green pigment chlorophyll, which breaks down with the diminishing sunlight of fall.

The word foliage in the passage meansA．season of year
B．type of chemical
C．mass of leaves
D．species of tree