Once children begin to read they acquire new vocabulary most naturally as the indirect result of reading difficult, challenging material. The major alternative -- direct studying of new vocabulary items -- is less natural; most people are not so inclined. From age six to age thirteen, children learn thousands of new words a year, mostly through reading; direct studying accounts for less than one-tenth of all new words learned. So it is bound to be inefficient for young students to learn new vocabulary by direct studying. Which one of the following, if true, most seriously weakens the argument? ['Reading difficult material sometimes leaves one confused as to the meaning of certain words.', 'Adults who habitually read difficult material tend not to learn many new words from doing so.', 'Experts disagree on the estimates of how many words children typically learn from age six to age thirteen.', 'Children from age six to age thirteen spend vastly much more time reading than they spend directly studying new vocabulary.']