A.

I am collecting TXT postcards.  

I have postcard more than fifty. -> I have more than fifty postcards. 

Because I like TXT.  ✔​

Reason is good at dancing. And good at singing. -> I like them because they are good at dancing and singing. 

I will buy a TXT album too.

B.

1. Wake up. ✔​

2. Look at me. This time is art class. -> Pay attention, please. This is Art class.

=> Corrected : The sentences are corrected grammatically. 

3.Wow!  ✔ (Punctuation)

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