Dave's ESL Cafe's Student Discussion Forums

[Chan-Seung Lee]

[asterix]

I think #1 is the answer you are looking for, but,actually, you could say all of them are correct, although they do not mean quite the same thing.

#2. could mean that volcanic rocks are useful in making building materials, for example, grinding them up, and mixing them with other materials.

#3 Means more or less the same thing i.e. Volcanic rocks are useful for building a certain product with which you then make roads etc..