题干
The upset mother thought to herself, “If only I ______ what's going on in my little boy's head right now.”
A：know
B：knew
C：have known
D：had known
答案(点此获取答案解析）
B
”、“ 
”分别表示不同元素的原子。下列对图示模型理解正确的是（ ）