Android studio无法运行低于21的API级别

Question

这是我的build.gradle：

  android {
        compileSdkVersion 25
        buildToolsVersion "25.0.0"
        aaptOptions {
            cruncherEnabled = false
            useNewCruncher = false

        }
        defaultConfig {
            applicationId "com.example.prmgclient"
            minSdkVersion 15
            targetSdkVersion 25
            versionCode 1
            versionName "1.0"

            testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"

        }

    }



  dependencies {
        compile fileTree(include: ['*.jar'], dir: 'libs')
        androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
            exclude group: 'com.android.support', module: 'support-annotations'
        })
        compile 'com.android.support:appcompat-v7:25.0.0'
  }

buildscript {
    repositories {
        jcenter()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:2.2.2'
    }

我的测试机器是android 4.4.2, API 19.Android studio 2.2.2.  我试图删除compile 'com.android.support:appcompat-v7:25.0.0'，但它没有用。 我该如何解决这个问题？