ПОМОГИТЕ!!!Complete the sentence. Use the PRESENT PERFECT or the PRESENT PERFECT

ПОМОГИТЕ!!!
Complete the sentence. Use the PRESENT PERFECT or the PRESENT PERFECT CONTINUOS. In the sentences, either form is possible.
The post office isnt far from here. I (walk) ____________________________ there many times.
Im tired. We (walk) _____________________ for more than an hour. Lets stop and rest a while.
Mr. Alvarez (work) __________________________ at the power company for fifteen years. He likes his job.
I (read) ______________________________ this chapter in my chemistry text three times, and still dont understand it!
My eyes are getting tired. I (read) _____________________________________ for two hours. I think Ill take a break.
Mrs. Jackson (teach) ________________________ kindergarten for twenty years. Shes one of the best teachers at the elementary school.
Debbie is writing a letter to her boyfriend. She (write) __________________ it since she got home from class. Its going to be a long letter!
I (write) my folks at least a dozen letters since I left home and came here.