Which of the following types of numbers are considered exact and therefore have an unlimited number of significant figures?

• A. Mass readings from a digital balance
• B. Volume readings from a graduated cylinder
• C. Counting numbers, such as the number of students in a classroom
• D. Measured values obtained using a ruler

Answer: (C)

Verified step by step guidance

1. Understand the difference between exact numbers and measured numbers. Exact numbers are values that are known with complete certainty and have an unlimited number of significant figures, while measured numbers have some degree of uncertainty.
2. Recognize that counting numbers, such as the number of students in a classroom, are exact because they represent a precise count without any measurement uncertainty.
3. Identify that mass readings from a digital balance, volume readings from a graduated cylinder, and measurements using a ruler are all measured values and therefore have limited significant figures due to instrument precision and measurement uncertainty.
4. Recall that exact numbers come from definitions, counts, or constants, and are not subject to measurement error, which is why they have unlimited significant figures.
5. Conclude that among the options given, only counting numbers are considered exact and have an unlimited number of significant figures.