Consider the following statements:Statement I: In the context of effect of water on rocks, chalk is known as a very permeable rock whereas clay is known as quite an impermeable or least permeable rock.Statement II: Chalk is porous and hence can absorb water.Statement III: Clay is not at all porous.Which one of the following is correct in respect of the above statements?

1. A

   Both Statement II and Statement III are correct and both of them explain Statement I

   Both II and III correct and both explain I. III fails: clay has porosity; its permeability is low.

2. B

   Both Statement II and Statement III are correct but only one of them explains Statement I

   Both correct, only one explains. Still needs III to be true.

3. C

   Only one of the Statements II and III is correct and that explains Statement I

   Only one of II and III is correct and that explains I. II holds: chalk is porous and can hold water, which is why it is permeable in Statement I. III’s ‘clay is not at all porous’ is the absolute that fails. Clay can be porous and still impermeable.

4. D

   Neither Statement II nor Statement III is correct

   Neither. Porous chalk is the explanation for I.

Summary. Official key is (c): only one of II and III is correct, and that one explains I. Chalk is porous and permeable; clay is impermeable but it is not ‘not at all porous’ — clay can be porous yet impermeable because pores are tiny. So III fails, II holds and explains I. (a)(b) keep III; (d) drops II.