math HELP!!

Which of the following is a prime number?

A.35 =7*5
B.49 =7*7
C.53
D.87 =29*3

Answer : C

Simplify the following algebraic expression.(要過程)

A. 3r + 4s - 2r+7s
=3r-2r+4s+7s
=r+11s

b. a+2b+3c+4a-6b-2c
=a+4a+2b-6b+3c-2c
=5a-4b+c

C. 2xy+3x-2y+4x-6y+5xy
=2xy+5xy+3x+4x-2y-6y
=7xy+7x-8y

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