在感到羞耻的时候，我们正确的方法是（   ） 

Dear Grandma and Grandpa,

    I miss you so much. Thank you for the new bike! I'm so excited! How did you know 1 is my favorite color?

    When I first 2 the box under the Christmas tree, I didn't know what it was. I thought it was 3 small to hold a bike. Then when I opened it, I saw that the bike was in small pieces. That's 4 the box was small!

    Dad and I put them 5 the next day. It was a lot of fun. 6 we finished, I got on the blue bike and 7 around the town. My old bike was too small for me. This one is just 8. It looks great, too! All 9 friends want one like this now. Dad said that I could take my 10 when we go to visit you in the spring. I can't wait.

Yours,

David

扦插时，保留有芽的插枝比较容易生根成活，这是因为（　　）

如果直线$ax+2y+1=0$与直线$x+y-2=0$互相垂直，那么a的值等于（　　）