how to know what's the cause of a package block?[solved]

[GNUtoo]

hello mDNSResponder is pulled as a dep and i can't emerge world or kdeaddressbook...

[ebuild N ] net-misc/mDNSResponder-107.6-r5 USE="gcj -debug -doc -java" 1,408 kB
[ebuild U ] kde-base/kdelibs-3.5.9-r4 [3.5.8-r4] USE="alsa branding cups fam jpeg2k kerberos openexr spell tiff -acl* -arts -avahi -bindist -debug -doc -kdeenablefinal -kdehiddenvisibility -legacyssl -lua -utempter -xinerama" 15,242 kB
[ebuild U ] kde-base/libkdenetwork-3.5.9 [3.5.8] USE="-arts -debug -kdeenablefinal -xinerama" 14,159 kB
[ebuild U ] kde-base/ktnef-3.5.9 [3.5.8] USE="-arts -debug -kdeenablefinal -xinerama" 0 kB
[ebuild U ] kde-base/libkcal-3.5.9 [3.5.8] USE="-arts -debug -kdeenablefinal -xinerama" 0 kB
[ebuild U ] kde-base/certmanager-3.5.9 [3.5.7-r1] USE="-arts -debug -kdeenablefinal -xinerama" 0 kB
[ebuild U ] kde-base/libkdepim-3.5.9 [3.5.8] USE="-arts -debug -kdeenablefinal -xinerama" 0 kB
[ebuild U ] kde-base/libkpimidentities-3.5.9 [3.5.7] USE="-arts -debug -kdeenablefinal -xinerama" 0 kB
[ebuild U ] kde-base/kontact-3.5.9 [3.5.7-r1] USE="-arts -debug -kdeenablefinal -xinerama" 0 kB
[ebuild N ] kde-base/kaddressbook-3.5.9 USE="-arts -debug -gnokii -kdeenablefinal -xinerama" 0 kB
[blocks B ] net-misc/mDNSResponder (is blocking net-dns/avahi-0.6.22-r1)
[blocks B ] net-dns/avahi (is blocking net-misc/mDNSResponder-107.6-r5)

[aceFruchtsaft]

Use the emerge -t switch to see which ebuild tries to pull in mDNSresponder. Maybe that ebuild has an USE flag you need to disable.

[GNUtoo]

thanks a lot it's a very usefull option...emerging with the avahi USE flag solved the problem...