∵

CM

=

1

3

CB

+

1

3

CA

∴

MA

=

CA

-

CM

=

CA

-(

1

3

CB

+

1

3

CA

)=

2

3

CA

 - 

1

3

CB

MB

=

CB

-

CM

=

CB

-(

1

3

CB

+

1

3

CA

) =

2

3

CB

-

1

3

CA

∴

MA

•

MB

=(

2

3

CA

-

1

3

CB

) • (

2

3

CB

- 

1

3

CA

)=

5

9

CA

 •

CB

-

2

9

|

CA

|2 -

2

9

|

CB

|2=

5

9

×|

CA

| ×|

CB

| ×cos60°-

2

9

(2

3

)2-

2

9

(2

3

)2

=

5

9

×2

3

×2

3

×

1

2

-

2

9

×12-

2

9

×12=

30

9

-

48

9

=-2

故选A