阅读理解
One sunny afternoon, I went for a walk on the beach. There I saw a special crab (螃蟹). It had two claws (螯). One claw was much bigger and longer than the other. I had never seen such a crab before. So on my way home from the beach, I walked into the library.
Luckily, I found a book named Come with Me. to the Edge of the Sea. It had many pictures of animals living near the sea. The pictures of male fiddler crabs (雄性招潮蟹) looked quite like the crab I found on the beach. I learned a lot about that kind of crab:
Sometimes g fiddler crab puts the small Claw across big one. It looks like it is playing the fiddle (violin) when it does this so some people call it the fiddler crab Sometimes it waves (挥动) its big claw It looks like it is saying hello. That's why other people call it the calling crab Fiddler crabs eyes are on long eye stalks (眼柄). They can put up their eye stalks to look around. It makes them look taller.
These crabs are very small. There are lots of other, bigger beach animals that want to eat them. They dig holes in the sand (在沙子里挖洞). The holes are their homes. They are safe inside them. Sometimes the sea water can bring food into their holes.
根据材料内容简要回答下列问题。
，每个编号代表下列物质中的一种物质，凡是用直线相连的两种物质均能发生化学反应。供填写的物质有：a.Na2CO3溶液；b.MgSO4溶液；c.BaCl2溶液；d.稀H2SO4；e.铁；f.Cu(NO3)2溶液。图中编号④代表的物质是 （ ）