鸡冠有两种性状﹣﹣玫瑰冠和单冠．若亲代个体都为玫瑰冠，后代个体中有玫瑰冠和单冠（R表示显性基因，r表示隐性基因）．则下列推断正确的是（    ）

已知：如图，B、C是线段AD上两点，且AB：BC：CD=2：4：3，M是AD的中点，CD=6cm，求线段MC的长．

     This is my family. I  1  a big family. My father is a  2 . He  3 in a hospital. My mother is a nurse.  4 works in the hospital, too. I have 5 uncle. He is my father's  6 . He drives a car. Susan is his wife(妻子). She is 7 aunt. She is a teacher and she works in a  8 . They have  9  kids. Bob is their son, and Amy is their  10 . We all love our family.

下表为中国、印度、美国、澳大利亚1997—1999年耕地利用情况，分析表中数据，回答下列各题。