Answer: CD (1st answer choice)
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Step-by-step explanation:
Matrix C has 3 rows and 2 columns. The size or dimensions can be written in shorthand as 3x2.
Matrix D has dimensions 2x1, aka 2 rows and 1 column.
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Write "3x2" and "2x1" side by side in that exact order.
Notice the inner '2's match up. This is important. We need the inner numbers to match so that matrix multiplication is possible. The outer values '3' and '1' will help form the dimensions of the final answer matrix. The final answer matrix will have dimensions 3x1
Luckily we don't need to compute the value of CD. We just need to use the justification mentioned in the previous paragraphs to conclude that matrix CD is possible.
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Something like BC is not possible because B is 3x2 and C is 3x2. We don't have a match of inner values this time. We can then rule out the answer choice "BC". The answer choices "DC" and "CB" are ruled out for similar reasoning.