The students were_______to learn that the new teaching methodat the meeting would be carried out.

A．pleased; put forward

B．pleasing; putting forward

C．pleased; putting forward

D．pleasing; put forward

A，B，C，D四种物质中，已知A单质，B为无色液体，B，C均为植物光合作用的原料，D为黑色固体，它们之间的转化关系如图所示（部分物质和反应条件已略去），其中①②为化合反应，③④为置换反应．则物质A为　  　；反应④的化学方程式为　                  　．