學習完小數(shù)乘法，小明想，能不能換個方法來計算？于是他這樣計算0.3×0.7（如下）。小剛想用小明的方法算一算2.1×0.09，下面的四種計算過程哪種是對的？說一說你的理由。
0.3×0.7
=3×0.1×7×0.1
=21×0.01
=0.21
（1）2.1×0.09
=21×0.1×9×0.01
=189×0.001
=0.189
（2）2.1×0.09
=21×0.1×9×0.1
=189×0.01
=1.89
（3）2.1×0.09
=21×0.01×9×0.01
=189×0.0001
=0.0189
（4）2.1×0.09
=21×0.1×9
=189×0.1
=18.9