it have been necessary 和 it is necessary 的区别?请问下句中it had been necessary用现在时为何不行?But rich planterswere few in the young county of Clayton,and,inorder to muster a full-strength troop,it had been necessary to raise more recruitsamong the sons of small farmers,hunters in the backwoods,swamp trappers,Crackersand,in a very few cases,even poor whites,if they were above the average oftheir class.

it have been necessary 和 it is necessary 的区别?请问下句中it had been necessary用现在时为何不行?
But rich planterswere few in the young county of Clayton,and,inorder to muster a full-strength troop,it had been necessary to raise more recruitsamong the sons of small farmers,hunters in the backwoods,swamp trappers,Crackersand,in a very few cases,even poor whites,if they were above the average oftheir class.