26．Even if there is no scientific proof yet ______ secondhand smoke causes cancer, there is no reason why nonsmokers should be forced to take this risk．
| A．which | B．that | C．when | D．Where |
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26．Even if there is no scientific proof yet ______ secondhand smoke causes cancer, there is no reason why nonsmokers should be forced to take this risk．
| A．which | B．that | C．when | D．Where |
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