Translation:Glass is cheaper than aluminum.
In the previous sentence the definite article before "silicon" was not accepted but here as in other exercises a material plus definite article was given as an alternative. That said in English the definite article is not used unless it is in relation to a specific example of the metal.
We are accepting these both with and without the definite article because without context the meaning can be interpreted in various ways. For example here if describing the costs of glass and aluminum in general we wouldn't use an article. But if we said: "Which door will you buy." we might use an article.
Yes I agree but it does sound odd in some instances, however it is a hard one to call.
Exactly. It's a hard one to call and as long as we get the Greek across we sort of loosen the reins. ;-)
is φθινο an alternative spelling fo φτινο - and if so then can φτ always be spelt as φθ?