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某种开花植物细胞中，基因P（p）和基因R（r）分别位于两对同源染色体上，将纯合的紫花植珠（基因型为PPrr）与纯合的红花植株（基因型为ppRR）杂交，F₁全开紫花，自交后代F₂中紫花：红花：白花=12：3：1。则F₂中表现型为紫花的植株基因型有：(   )

A

    Look! This is a picture of my teacher's family. This is Mr. Smith.  He  is my English  teacher. His wife is a doctor. She is nice. They have a son and a daughter. The boy's  name is Sam and the   girl's name is Kate. They are students. They are in the same school. They go to school on their bikes. Look! The son is like his mother. The daughter is like her father. Now they are all in China and can speak Chinese. They love China.

如图所示，质量为M的木板放在水平桌面上，一个质量为m的物块置于木板上．木板与物块间、木板与桌面间的动摩擦因数均为μ．现用一水平恒力F向右拉木板，使木板和物块体共同向右做匀加速直线运动，物块与木板保持相对静止．已知重力加速度为g．下列说法正确的是（   ）