A：①②
B：③④
C：②③
D：①④
-- Anna, could you lend me this book, please?
-- I'm sorry. I it for only two days, and I haven't finished it yet.
根据质量守恒定律解释下列问题：
蟾酥是蟾蜍表皮腺体的分泌物，干燥后可以入药．从多样性的价值来看，这属于（ ）
2.8＋5.6=____ 12.8－3.9=____
