煤是一种固体可燃有机岩，主要由（）、遗体经生物化学作用，埋藏后再经地质作用转变而成。俗称煤炭。
参考答案：植物
By the time Jack returned home from England, his son. from college.
A．graduated
B．has graduated
C．had been
D．had graduated
TYJL-Ⅱ型计算机联锁系统大量信息采集不到是什么原因？
eturned home from England, his son. from college.