Take the midpoint of BC as e and connect AE and DE, then the dihedral angle formed by BCD and ABC is to find ∠BED,

AB = AC = mathematical formula,

∴△ABC is an isosceles triangle;

∫E is the midpoint of BC;

∴AE⊥BC, BE= mathematical formula BC =1;

In right angle △ABE, AE2 = AB2-BE2 is obtained by Pythagorean theorem;

∴AE= mathematical formula;

∵ Tripartite and bottom ABC congruence; ∴DE=AE= mathematical formula;

∫△DBC?△ABC； ∴DB=AB= mathematical formula;

∫△ABC?△ bad; ;

∴ad=bc=2； So △ Abe's trilateral AE=DE= mathematical formula, AD = 2；; AE2+DE2 = AD2；

So AE ⊥ Germany; ∴∠DEA=90

Therefore, the dihedral angle formed by BCD plane and ABC plane is 90;

So choose C.

Comments: The knowledge examined in this question is the solid geometry synthesis problem related to dihedral angle, in which ∠BED is the plane angle of dihedral angle formed by BCD and ABC, and transforming dihedral angle problem into triangle problem is the key to solve this problem.