Which sentence best states the main idea?

A lichen is two organisms living as one, and that deal brings both power and cost. (The passage defines the partnership, shows what each partner gives, then names its two costs: slow growth and exposure to the air.)

According to the passage, what does the fungus contribute to the partnership?

It grips the rock, holds water, and shields its partner from strong light. (The second paragraph lists exactly these three jobs for the fungus; making sugar from sunlight is the alga's job.)

Lichens have disappeared from the trees of a city park. What can you reasonably infer?

The air in that area is probably polluted. (The text says lichens absorb whatever the air carries and vanish first where pollution is heavy, and that scientists read their absence as a warning.)

In this passage, 'shield' means:

protect from something harmful (The fungus shields its partner from strong light, that is, it protects the alga from light that would harm it.)

Two Lives, One Body: Level 3 Reading Comprehension Questions

The grey-green crust on an old stone wall looks like a single plant. It is not a plant at all, and it is not a single organism. A lichen is a partnership: a fungus that builds the body, and an alga or a bacterium that makes food inside it.

The division of labour is clear. The fungus cannot make its own food, but it can grip bare rock, hold water, and shield its partner from strong light. The alga has no roots and no shelter, but it can turn sunlight into sugar. Together they live where neither could live alone, on gravestones, on bark, on stone in the high mountains.

The cost of the arrangement is speed. Many lichens grow less than a millimetre a year, and some patches on Arctic rock are centuries old. The cost is also exposure. A lichen has no waxy skin and no way to shut itself off from the air, so it absorbs whatever the air carries, clean or dirty. Where pollution is heavy, lichens thin out and vanish before most other life shows harm. Scientists read that absence as a warning.

Comprehension questions