已知在等差数列an中3a2=7a,a1>0,则下列说法正确的是（）

题干

A： $a_{11} > 0$

B： $s_{10}$ 为 $s_{n}$ 的最大值

C：d>0

D： $s_{4} > s_{16}$

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B

School uniforms are common in primary and secondary schools in many countries. Here’s a list of countries below.

	In Brazil, students wear uniforms in private schools. But in public schools, students don’t have to wear uniforms although they might have one. In general, the uniforms are a pair of blue pants and a white T-shirt with the school symbol printed on it.
	School uniforms are used in Cuba. All students wear school uniforms with the color as a symbol of grade level. Children also wear scarves because they are young pioneers(少先队员).
	Japan introduced school uniforms in the late 19^th century. Today, school uniforms are very common. Many boys wear white shirts, short pants and caps. Girls’ uniforms might include a gray pleated (褶)skirt and a white blouse.
	In Singapore, all students must wear uniforms. Though the colors are different from school to school. The normal uniform for boys is shorts or long pants, with a shirt (often white), while girls wear white pinafores(连衣裙) or skirts.