28. The phrase "account for" in line 22 is closest in meaning to (A) reduce (B) explain (C) combine with (D) list all of

29. The passage supports which of the following conclusions? (A) Clay deposits are only found deep in the Earth. (B) If clay contains too much iron it will melt when fired. (C) Only certain types of clay are appropriate for making china teacups. (D) If sufficient heat is applied, all clay will become nonporous.