19．Vitamnin D helps our bodies make a special chemical，and this special chemical，----，

　 affects our brains，making us feel happy．

　 A．in turn　　 B．in particular　　 C．in advance D．in short

20 Helen devoted herself--to her research and became a top scientist in the field of applied

　physics．

　 A．seriously　　　　 B．extremely　　 C．entirely D．strictly

18．Everyone must have known the death of the waitress，--?

　 A．must they　　 B．mustn't they　　 C．haven't they D．have they