Answer:
C. The units digit will be different
Explanation:
Adding 3 tens is the same as adding 30 to a number. If we're looking for which will NEVER happen we can eliminate answers if we find and example of each wrong option. Just in case, units digit just means digit in ones place.
A. is wrong because 9970 + 30 = 10,000 which is a 5 digit number.
B. is wrong because because 1000 + 30 = 1030 which is a 4 digit number.
C. is correct because the units digit of 30 is 0 so no matter how many times you add 30 the units digit will remain the same.
D. is wrong because 1070 + 30 = 1100 which has a different hundreds digits.