We thank you for your letter of May 17 and the（）catalogue.

A.sent

B.enclosed

C.given

D.presented

饮水氟化预防龋病的适宜氟浓度是

A．0.1～0.3mg/L

B．0.4～0.6mg/L

C．0.7～1.0mg/L

D．1.1～1.3mg/L

E．1.4～1.6mg/L