At the beginning of the 20th century, many people thought that the family was falling apart—__________, they thought it was dying． （ ）
例如，十六进制的14对应的十进制的数为20，用十六进制进行表示C+F=1B．19-F=A，18÷4=6，则A×B= ( )
双受精作用指的是（ ）
