My father's day

    My father is a teacher. He teaches Chinese. He likes his job and his students. And the students like him too. He goes to work from Monday to Friday. He does not work on Saturday and Sunday. Every day, he gets up at five thirty. At six, he has breakfast. Then he goes running. He goes to school at seven. He has lunch at school. He goes home at five in the afternoon. At six o'clock, he has dinner. He doesn't watch TV. He likes reading（读）books with me. He goes to bed at nine thirty.

木板与水平面之间的夹角θ，发现当θ=30^o和θ=45^o时物块A所受的摩擦力大小恰好相等，则物块A与木板之间的动摩擦因数大小为（    ）