486﹣186=____ 250+250=____ 0.25×0.8=____ 4.2÷6=____

$\frac{1}{2}+\frac{1}{3}$ =____  $\frac{1}{3}-\frac{1}{5}$ =____ 24× $\frac{3}{8}$ =____ 21÷ $\frac{3}{7}$ =____

人在恐惧、紧张时，在神经系统的支配下，肾上腺髓质释放的肾上腺素增加，该激素可作用于心脏，使心率加快。下列叙述正确的是(  )

    San Francisco is a wonderful vacation destination that offers many different things to see and to do. The best way for a traveler to get a feel for the city is to take one of the many different tours there.

    Foot. When touring the city by walking, you aren't going to walk much. There are far more benefits to touring the city on foot, though. This kind of tour allows you to see many of the buildings. It usually makes you see a more localized neighborhood level.

    Water. The waters of San Francisco Bay have played an important role in the city's development over the last century. Touring San Francisco by water is a completely unique way for you to see this wonderful city.

    Bus. If you want to get a great feel for the layout of the city, one of the best things you could do is book a tour through our company. A bus tour of San Francisco is one of the most complete ways to experience the city.

    Air. While it is the most expensive way to see San Francisco, it is also one of the most unique and thrilling ways to see the city. Seeing the city from high above allows you to get a good feel for the layout of the city as tour guides point out landmarks and attractions. If you decide to tour San Francisco from the air, you'll get memories that you won't forget forever.

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已知⊙O₁与⊙O₂的半径r₁=2、r₂=4，若⊙O₁与⊙O₂的圆心距d=5．则⊙O₁与⊙O₂的位置关系是____ .