— is it from your home to your school?
— It is about twenty minutes by bike.
A:How far
B:How long
C:How many
D:How much
沈括在《梦溪笔谈》中记载了“以磁石磨针锋”制造指南针的方法,磁针“常微偏东,不全南也”.他是世界上第一个指出地磁场存在磁偏角的人,比西方早了400年.关于地磁场,下列说法中正确的是( )