C. will have
This is a conditional sentence of type 1. It expresses a real possibility in the future. The structure is:
If + present simple, future simple
So, if we meet at 9:00, we will have plenty of time to talk.
D. to concentrate
This sentence uses the infinitive form "to concentrate" after the adjective "difficult". It's a common pattern in English to use an infinitive after adjectives like "difficult," "easy," "interesting," etc.

 

dap an cau 20 : B
cau 21 D