it's good for you to keep () ,when something difficult needs( )A trying;to doB trying;doingC to try;to doD to try;to be done

it's good for you to keep () ,when something difficult needs( )
A trying;to do
B trying;doing
C to try;to do
D to try;to be done

trying
doning=to be done

?

没给选项啊

B