An orange NARROW LANES sign has two tabs beneath it: one reading TRUCKS USE CENTRE LANE, the other reading FOR 12 KM. What are the two tabs each doing?

Correct answer: C. One sends trucks to the widest lane at this site; the other says the narrow-lane condition lasts 12 km

One assigns a lane, the other tells you how long to expect it. Book 7 says the NARROW LANES sign warns drivers "on multi-lane highways that they are approaching a work area where lane widths have been reduced due to work operations." The TRUCKS USE CENTRE LANE tab is used "where lanes have been narrowed and the designated lane is the widest or preferred lane for trucks to use" — the named lane changes with the site, so read it rather than assume. The distance tab "may be used to advise drivers of the length of highway for which the narrow lane condition exists", and the manual notes it "may be used with other signs where it is considered desirable to advise drivers of the length of a given work zone or condition." So a distance tab under a work-zone sign is telling you how far the condition runs, not how far away it starts.

Source: MTO Driver's Handbook: Ontario Traffic Manual, Book 7 — Temporary Conditions (April 2022), Section 4.2.8.5 — Narrow Lanes Sign (TC-11) and Trucks Use Centre Lane Tab Signs (TC-11t, TC-11tA)