a, Thank you very much for your letter. I'm sorry that I haven't written to you earlier.
b, The book you sent me on my last birthday is very interesting. It helps me a lot in my daily life.
c, This week my family has more free time, so we decided to go on a picnic to Quy Nhon Beach on Saturday.
d, Would you like to go with us? I will phone you on Thursday for details.