\( 21 \div \square=7 \). (A) 1 (B) 3 (C) 7 (D) 9

To solve \( 21 \div \square = 7 \), we can think of it like this: if we multiply the number we are looking for (the square) by 7, we should get 21. So, we can express this as \( \square = 21 \div 7 \). When we perform the division, we get \( \square = 3 \). Therefore, the correct answer is (B) 3. Moreover, division can be viewed as splitting a group into equal parts, which is a concept that even ancient civilizations utilized for trading and fair distribution. So, the next time you divide, remember that you're tapping into a practice as old as humanity itself! When it comes to avoiding mistakes, remember that it's easy to confuse multiplication and division. An effective tip is to always check your work by doing the reverse operation. If \( 3 \times 7 = 21 \) checks out, then you’ve got it right! Happy calculating!