A 改为肯定句 It is OK to leave yourself at home by myself.这是固定句型。that,this 都是指代某一个。如This/That pen is mine.而不能说It pen is mine.this指近物，“that”指远物，“it”没有远近之分。如：This is a computer.这是一台电脑。（在近处）That is a bike. 那是一辆自行车。（在远处）在回答“this”和“that”作主语的疑问句时，常用“it”代替“this”和“that”。如：－Is this/that a car?这/那是一辆小汽车吗？－Yes, i
