It must have rained last night, ________ the ground is wet now. 

 It puzzles the scientists _____ some mammals produce their young _____ others lay eggs. 

 We had not waited __________ she came.

—What upsets him?—________ his computer is out of order.

Green economy offers a new model ______ people could both protect the climate and develop the economy, according to what Li said at the International Cooperative Conference on Green Economy and Climate Change.