观察量筒中液面刻度时，要使视线与液面相平，量筒水银面是凸形的，观察时要（  ）

根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。

    Canada is the second largest 1 in the world. It is over 7,000 kilometres from the 2 coast (海岸）to the east. It 3 six time zones. When it is 9 a.m. in Vancouver on the west coast, it is 1:30 p.m. in St John's on the east coast.

    English and French are the two main 4 in Canada. About 60 percent of its people 5 English, and about 25 percent speak French. More than 70 percent of its population 6 in cities near the US border (边境).

    Ottawa is the capital. Every year millions of tourists go there to 7 museums and take part in the cultural 8. Toronto is the largest city of the country with about 2.5 million people. Montreal is the second biggest French-speaking city in the world.

The maple leaf (枫叶）is the 9 symbol of Canada. The Canadian 10 has a red maple leaf on a red and white background.

胚胎干细胞的特点是（　　）