Nathaniel answered the following subtraction problem incorrectly 16.43-1.043=6.00

Part A
fill in blank to describe nathaniel error, and how he could have known that his difference was incorrect.
when trying to solve this problem nathaniel did not line up the decimal according to___. Nathaniel could have estimated to see that answer was incorrect. he could have subtracted the whole number from the whole number to see that is different should be about.

(I already have part B)​