这个问题,前面解答过了。
这是被动语态:be done by sb.
that has been reinforced by the widespread introduction of refereeing, first by national journals in the 19th century and then by several local geological journals in the 20th century.
“结果被......加强,先被......加强,然后又被......加强”。后两个“by”,进一步解释并列举第一个“by”。
再举一个简单的例子:
This book has been read by two boys, first by Tom and then by John.
后两个by...进一步说明第一个by...的具体内容。
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