A:To avoid being see
B:Avoiding see
C:Avoid being see
D:Avoided seeing
一共组成了____个长方形.
如图所示,袋鼠妈妈把小袋鼠放进育儿袋中后,在草地上向前跃进。说小袋鼠是静止的,选取的参照物是( )