Which of the examples of a situation where zero is not considered to be a significant digit?

Zero is not considered a significant digit in situations where it serves as a placeholder. For example, in the number 0.0045, the leading zeros before the 4 do not count as significant digits; only the digits 4 and 5 are significant. Similarly, in the number 1000, if no decimal point is present, the trailing zeros may not be considered significant, depending on the context or notation used.