人体缺少某种元素，会引起甲状腺肿大。则该元素是
A：钙
B：碘
C：硒
D：铁
人类在生物圈中扮演的角色是生产者．
__________ of students are in the room and nine ________ of them are girls.
Be ____ (care) of the dog. Don't get too close to it.
