Chinese public buildings erected under a construction code of the Sung dynasty have withstood earthquakes well because the white cedar used has four times the tensile strength of steel and the timber frame, incorporating many joints and few nails, is flexible. (A) used has four times the tensile strength of steel and the timber frame, incorporating (B) used in them has four times the tensile strength of steel has and the timber frame, incorporating (C) that was used in them has four times the tensile strength steel has, and the timber frame, incorporating (D) that was used has four times as much tensile strength as steel, and the timber frame incorporates (E) that was used has four times the tensile strength steel does, and the timber frame incorporates SC73561.01 JOURNAL ARTICLE Chinese Building Standards in the 12th Century Else Glahn Scientific American Vol. 244, No. 5 (May 1981), pp. 162-173 Published by: Scientific American, a division of Nature America, Inc. https://www.jstor.org/stable/24964424 Page Count: 12 The first of these factors is that pound for pound white cedar (the wood most commonly used for construction in traditional Chinese buildings) has about four times the tensile strength of steel , and its resistance to compression is about six times that of concrete. A second factor is that a timber frame incorporating many joints and few nails is surprisingly flexible.

Yes, C does mention "the timber frame is flexible" as an independent clause and this is what the explanation also says: the timber frame's flexibility is mentioned simply as a separate fact not falling under the scope of "because" . To ensure that timber frame's flexibility does not come across as a separate fact, the sentence could have been: ...because the white cedar has four times the tensile strength...and because the timber frame is flexible.