What is the passage mainly about?

How carbon-14 dating works, and why its results must be corrected. (The first two paragraphs explain the method; the third explains the complication and the correction that follows from it.)

Why does the method stop working after roughly fifty thousand years?

So little carbon-14 remains that there is nothing to measure. (The text states that after roughly fifty thousand years so little remains that the method runs out.)

A sample holds about one quarter of the carbon-14 it began with. Roughly how old is it?

About 11,500 years (Half is lost in about 5,730 years and half of the rest in the next 5,730, so a quarter left means about two such steps, near 11,460 years.)

In this passage, 'constant' means:

staying the same over time (The text says the proportion has not been constant, then adds that it has drifted, so constant means unchanging.)

A Clock In Dead Wood: Level 4 Reading Comprehension Questions

High in the atmosphere, cosmic rays strike nitrogen atoms and turn a few of them into carbon-14, a rare and unstable form of carbon. It mixes into the air as carbon dioxide, plants take it in, animals eat the plants, and every living body ends up holding about the same proportion of it as the air does.

Death closes that account. Nothing new comes in, and the carbon-14 already present breaks down at a fixed rate: half of it is gone in about 5,730 years, half of the remainder in the next 5,730, and so on. Measure how much is left in a piece of charcoal, bone, or cloth, and you can work out how long ago the exchange with the atmosphere stopped. After roughly fifty thousand years so little remains that the method runs out.

There is a complication. The proportion of carbon-14 in the air has not been constant; it has drifted with changes in the sun and, more recently, with the burning of fossil fuels. So raw measurements are corrected against samples of known age, tree rings above all, before a date is announced. The clock is real, but it has to be set.

Comprehension questions