7. Mr. Pei Wenzhong , a famous Chinese Paleolithic archaeologist, discovered the first skull of a Peking man on December 2, 1929.

In Hu Wenzhong's theory of An Iceberg of Culture, only a small part of culture is______ and______ while most of it is invisible and intangible. A: unreachable B: visible C: superficial D: unforgetable