实验室的烧杯中有一定量盐水，第一次加入适量水后，烧杯中盐水浓度为3%，第二次再加入同样多水后，烧杯中盐水浓度为2%，则第三次加入同样多的水后盐水浓度为多少？（ ）

A.12

B.18

C.9

D.6

A.12

B.13

C.14

D.15

A.30

B.45

C.60

D.72

A.540

B.580

C.600

D.640

A.56

B.64

C.80

D.120

A.20

B.30

C.40

D.50

A.3/10

B.2/5

C.1/2

D.3/5

A. an; the

B. the; a

C. the ;/

D. / ; a

A. writing

B. to write

C. written

D. being written

A. In reply to

B. In addition to

C. In charge of

D. In place of