“凡尔赛—华盛顿体系”形成于（    ）

一架飞机起飞后不久遭到鸟击，致使飞机发生故障，下流说法正确的是（　　）

    Alice is a white dog. She is two. Bob is a brown dog. Bob and Alice are good friends. Look at the two dogs.

Bob: Good morning!

Alice: Good morning!

Bob: What's this?

Alice: It's a jacket. It's my sister's. Oh, my sister is Grace.

Bob: What's that?

Alice: It's a book. It's Miss Miller's book. She is an English teacher.

Bob: Well, dogs don't like books.

在平面直角坐标系xOy中，过原点O及点A（0，2）、C（6，0）作矩形OABC，∠AOC的平分线交AB于点D．点P从点O出发，以每秒$\sqrt{2}$个单位长度的速度沿射线OD方向移动；同时点Q从点O出发，以每秒2个单位长度的速度沿x轴正方向移动．设移动时间为t秒，当t为____时，△PQB为直角三角形．