From the abstract, what is the method of pervious study？（)

A. Promising laboratory test results still need to be confirmed by clinical studies before clinical recommendation could be given

B.Clinical trials with resin-bonded restorations that had no or only limited mechanical retention and were made from alumina or zirconia ceramic were identified using an electronic search in PubMed database.

C.Most original laboratory research reports on bonding to zirconia ceramics start in their introduction with a statement pointing more or less out that achieving a reliable and predictable bond to this ceramic would be a major problem in dentistry

D.No clinical trials with alumina ceramic IRFDFs or with RBFDPs made from densely sintered alumina ceramic were foun