The wavelength of a wave is the distance between

a. two consecutive crests. B. the crest and the following trough. C. the rarefaction and the following compression. D. the amplitude and the normal position.

C. the rarefaction and the following compression

Errors can come from various sources: the obtainer of the data, the measuring instrument, the setting and etc. Errors are what makes your measurement invalid and unreliable. There are two types of error which is called the systematic error and the random error. Each error has different sources. Words that were mentioned - invalid and unreliable are very important key aspects to determine that your measure is truly accurate and consistent. Some would recommend using the mean method, doing three trials in measuring and getting their mean, in response to this problem.