Please consider the highlighted part of your text:
[quote="OlegC"]Can you please help me with this:
1. The work of mathematician Roger Penrose in the early 1970s, on the geometry of <
WHAT ARE CALLED aperiodic tiles, turned out to describe> the architecture of a previously unknown class of crystals.
(correct A) what are called aperiodic tiles, turned out to describe
2. The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people <has been established by carbon-14 dating, but
what is much more difficult to determine are> the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.
(A) has been established by carbon-14 dating, but what is much more difficult to determine are
(correct B) has been established by carbon-14 dating, but WHAT is much more difficult to determine IS
Here is an excerpt from OG explanation for the second question: “Two instances of subject-verb agreement must be observed in this sentence: The PERIOD ... HAS been established and WHAT is much more difficult to determine ... IS. Both clauses have SINGULAR SUBJECTS and must have singular verbsâ€
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