“根由”怎么读

“根由”的英语可以翻译为：cause，origin，reason ...

他熬费苦心地描绘了这次野餐的根由.Laboriously did he describe the genesis of the picnic.大概这就是我和沙子结下不解之缘的根由.That's probably why I have an affinity with sand.这样,他力气的根由人还是不知道.So his strength was not known....